Everytime I restart my mac, I have to reinstall firefox, when i click it on the dock, a question mark shows up on the firefox icon, then i have to reinstall it, can anyone tell me why its like this, and how to fix it ?
It sounds like you're not in fact installing Firefox at all, but are rather running it from the Disk Image (.dmg file) you downloaded. You need to drag the Firefox icon from the Disk Image into the Applications folder on your hard disk. Then you can eject the Disk Image by dragging its icon from the Desktop to the Trash.
This goes with anything your download. You will open the .dmg file (usually) and it will in turn open up a white "folder." If you open and use the application from that white "folder" then you are just using a copy of the app. and must indeed drag the hole application to your application folder. From there you can then drag it to your dock.