Warning:
1. by NO means these are THE ONLY compatible extensions. I only went through 350 out of 2000 extensions, and I only pick ones that has general appealing to some users. I might do rest of them later
2. by NO means these will be THE ONLY compatible extensions, I only chose the ones that are compatible with beta 4 and above, AFAIS, 95% of the extensions are compatible with some versions of firefox 3, which indicates that most extensions WILL be compatible with firefox 3 eventually.
3. I did NOT check the compatibility of these extensions with firefox 2.
4. If you already have many extensions you like, goto https://addons.mozilla.org/en-US/firefox/ and check for yourself
These are the extensions I found (so far) interesting, and compatible with beta 4 or beta5pre
https://addons.mozilla.org/en-US/firefox/addon/5410 text shadow add CSS text shadow support to firefox.
https://addons.mozilla.org/en-US/firefox/addon/6227 oldbar change back the url dropdown list to firefox 2 style.
https://addons.mozilla.org/en-US/firefox/addon/4429 Secure login magic wand button (like in opera) for firefox
https://addons.mozilla.org/en-US/firefox/addon/5641 Quick TransLation in-browser auto pop-up dictionary for selected text
https://addons.mozilla.org/en-US/firefox/addon/5712 search all split browser window for searching with multiple search engine.
https://addons.mozilla.org/en-US/firefox/addon/5791 flagfox show national flag for the website you are visiting
https://addons.mozilla.org/en-US/firefox/addon/614 save image in folder context menu save different images to pre-configured folders
https://addons.mozilla.org/en-US/firefox/addon/6665 Prism convert webpage to web app
https://addons.mozilla.org/en-US/firefox/addon/3780 faviconTab allow only favicon displaying on tab to save space.
https://addons.mozilla.org/en-US/firefox/addon/4882 TabScope show preview thumbnail when move mouse over a tab
https://addons.mozilla.org/en-US/firefox/addon/1433 extended status bar show extended info at status bar, page loading speed, loaded bytes, # of images in page, % of page loaded etc.
https://addons.mozilla.org/en-US/firefox/addon/2010 Data Analytics DataAnalytics is a Firefox extension that enables importation, manipulation, analysis and graphing of data
https://addons.mozilla.org/en-US/firefox/addon/6521 status bar scientific calculator
https://addons.mozilla.org/en-US/firefox/addon/139 Image zoom zoom image in page, whichever you want, or all of them
https://addons.mozilla.org/en-US/firefox/addon/2871 url fixer auto fix common typo in url bar
https://addons.mozilla.org/en-US/firefox/addon/1557 QuickProxy status bar switch on/off of proxy
https://addons.mozilla.org/en-US/firefox/addon/1951 Fission progress bar in url bar, like safari, with more colors you can choose.
https://addons.mozilla.org/en-US/firefox/addon/3650 FAYT enable full featured find as you type bar
https://addons.mozilla.org/en-US/firefox/addon/1730 scribefire in-browser blog editor
https://addons.mozilla.org/en-US/firefox/addon/5890 Tree Style Tabs arrange tabs with tree structure, or move tab bar around, or auto hide it.
https://addons.mozilla.org/en-US/firefox/addon/6033 Magic's Video - Downloader
https://addons.mozilla.org/en-US/firefox/addon/2032 yahoo toolbar
https://addons.mozilla.org/en-US/firefox/addon/5721 Fast dial speed dial for firefox, only more places
https://addons.mozilla.org/en-US/firefox/addon/3559 quick restart
https://addons.mozilla.org/en-US/firefox/addon/722 No Scripts control javascript, java in webpage
https://addons.mozilla.org/en-US/firefox/addon/2848 passwords exporter easily syncing among all your computers
https://addons.mozilla.org/en-US/firefox/addon/1810 firefox showcase thumbnail preview of opened tabs
https://addons.mozilla.org/en-US/firefox/addon/748 Greasemonkey user define style sheet, javascript, alter webpage as you see fit (plus you know how, or download pre-configured user.js)
https://addons.mozilla.org/en-US/firefox/addon/6437 about icecream
https://addons.mozilla.org/en-US/firefox/addon/6543 Nightly Tester Tool for all non-compatible extensions, this extension fixes the compatibility issue for you (be warned, try not to use TOO ancient, non compatible extensions. This extension isn't magic bullet.)
1. by NO means these are THE ONLY compatible extensions. I only went through 350 out of 2000 extensions, and I only pick ones that has general appealing to some users. I might do rest of them later
2. by NO means these will be THE ONLY compatible extensions, I only chose the ones that are compatible with beta 4 and above, AFAIS, 95% of the extensions are compatible with some versions of firefox 3, which indicates that most extensions WILL be compatible with firefox 3 eventually.
3. I did NOT check the compatibility of these extensions with firefox 2.
4. If you already have many extensions you like, goto https://addons.mozilla.org/en-US/firefox/ and check for yourself
These are the extensions I found (so far) interesting, and compatible with beta 4 or beta5pre
https://addons.mozilla.org/en-US/firefox/addon/5410 text shadow add CSS text shadow support to firefox.
https://addons.mozilla.org/en-US/firefox/addon/6227 oldbar change back the url dropdown list to firefox 2 style.
https://addons.mozilla.org/en-US/firefox/addon/4429 Secure login magic wand button (like in opera) for firefox
https://addons.mozilla.org/en-US/firefox/addon/5641 Quick TransLation in-browser auto pop-up dictionary for selected text
https://addons.mozilla.org/en-US/firefox/addon/5712 search all split browser window for searching with multiple search engine.
https://addons.mozilla.org/en-US/firefox/addon/5791 flagfox show national flag for the website you are visiting
https://addons.mozilla.org/en-US/firefox/addon/614 save image in folder context menu save different images to pre-configured folders
https://addons.mozilla.org/en-US/firefox/addon/6665 Prism convert webpage to web app
https://addons.mozilla.org/en-US/firefox/addon/3780 faviconTab allow only favicon displaying on tab to save space.
https://addons.mozilla.org/en-US/firefox/addon/4882 TabScope show preview thumbnail when move mouse over a tab
https://addons.mozilla.org/en-US/firefox/addon/1433 extended status bar show extended info at status bar, page loading speed, loaded bytes, # of images in page, % of page loaded etc.
https://addons.mozilla.org/en-US/firefox/addon/2010 Data Analytics DataAnalytics is a Firefox extension that enables importation, manipulation, analysis and graphing of data
https://addons.mozilla.org/en-US/firefox/addon/6521 status bar scientific calculator
https://addons.mozilla.org/en-US/firefox/addon/139 Image zoom zoom image in page, whichever you want, or all of them
https://addons.mozilla.org/en-US/firefox/addon/2871 url fixer auto fix common typo in url bar
https://addons.mozilla.org/en-US/firefox/addon/1557 QuickProxy status bar switch on/off of proxy
https://addons.mozilla.org/en-US/firefox/addon/1951 Fission progress bar in url bar, like safari, with more colors you can choose.
https://addons.mozilla.org/en-US/firefox/addon/3650 FAYT enable full featured find as you type bar
https://addons.mozilla.org/en-US/firefox/addon/1730 scribefire in-browser blog editor
https://addons.mozilla.org/en-US/firefox/addon/5890 Tree Style Tabs arrange tabs with tree structure, or move tab bar around, or auto hide it.
https://addons.mozilla.org/en-US/firefox/addon/6033 Magic's Video - Downloader
https://addons.mozilla.org/en-US/firefox/addon/2032 yahoo toolbar
https://addons.mozilla.org/en-US/firefox/addon/5721 Fast dial speed dial for firefox, only more places
https://addons.mozilla.org/en-US/firefox/addon/3559 quick restart
https://addons.mozilla.org/en-US/firefox/addon/722 No Scripts control javascript, java in webpage
https://addons.mozilla.org/en-US/firefox/addon/2848 passwords exporter easily syncing among all your computers
https://addons.mozilla.org/en-US/firefox/addon/1810 firefox showcase thumbnail preview of opened tabs
https://addons.mozilla.org/en-US/firefox/addon/748 Greasemonkey user define style sheet, javascript, alter webpage as you see fit (plus you know how, or download pre-configured user.js)
https://addons.mozilla.org/en-US/firefox/addon/6437 about icecream
https://addons.mozilla.org/en-US/firefox/addon/6543 Nightly Tester Tool for all non-compatible extensions, this extension fixes the compatibility issue for you (be warned, try not to use TOO ancient, non compatible extensions. This extension isn't magic bullet.)
. I got the last build and works well with beta 4. Sadly b4 crashes more than 3 :-(