What is this?

http://uk.answers.yahoo.com/question/index?qid=20100401141538AAd7GBA
http://forums.mozillazine.org/viewtopic.php?f=38&t=631751&start=0&st=0&sk=t&sd=a

 

weird, never seen that before.

 

Summarizing the two useful links GGJstudios gave, that problem is what happens when, essentially, Firefox (or any other browser--I've seen it happen in Safari too) misinterprets a web page as a downloadable file.

One cause of this is just a badly-configured or misbehaving web server--simplifying, the document is being sent with the wrong identifying information, so the browser thinks it's some other type of file. Once in a rare while I'll run across one of these, but unless it's one specific site that you use that is having this problem, that's probably not your issue if it's happening that regularly.

The other cause, which sounds much more likely in your case, is that some Firefox add-on you've installed is causing Firefox to get confused about some files, resulting in it trying to save them instead of displaying them as a web page.

At one point ClickToFlash was causing Safari to do this regularly with Youtube, for example. The solution would be to disable all your FF add-ons, then re-enable them one at a time until you determine which is causing the problem. And/or make sure all your add-ons (and Firefox itself) are fully up to date.