See these articles for some suggestions:

• https://support.mozilla.com/kb/Firefox+has+just+updated+tab+shows+each+time+you+start+Firefox
• https://support.mozilla.com/kb/How+to+set+the+home+page - Firefox supports multiple home pages separated by '|' symbols
• http://kb.mozillazine.org/Preferences_not_saved

Thanks for your effort! But this doesn't help. My home page wasn't altered, it's the same as I put it. I'll try to reinstall firefox :(

Did you check for problems with the file prefs.js?

• http://kb.mozillazine.org/Preferences_not_saved

It didn't help! I tried everything what is written in this solution and nothing. :( Now I get 4 tabs open on start, annoying as hell! And every time I open firefox is checking compatibility for add ons. I will give time some more time to resolve this, and if nothing I', switching back to Iexplorer or Google chrome.

I have the same problem, it started in FF3, but it's still here in FF5, I have searched a lot and it seems like there is a good amount of people that have this problem and can't solve it.

EDIT: I found a solution, look beneath

Ok I found a solution that worked for me, I simply dragged the user.js and the user.js.BAK to the desktop, before that I had done the same with the prefs.js and prefs.js.BAK, but that didn't help, so it's apparently the user file that was an error with.

It was the link from cor-el that helped me on the way http://kb.mozillazine.org/Preferences_not_saved It's at the bottom titled User.js file locks certain preferences. This article can also help https://support.mozilla.com/en-US/kb/Preferences%20are%20not%20saved

It have still not made a new user.js file, but everything runs fine, hope this can help others to.