Try resetting your home page.

• see how to set the home page

There is a possibility something is causing that problem such as corrupt or multiple sesionstore files but lets assume the simplest problem and solution first.

The Beta or Aurora channels are probably better suited to general use than the Nightly channel.

See also 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

I have a VERY similar problem, but I dont open any site on FF 5 startup. When I go to a site, it keeps showing the "loading" icon even if the page seems compleatly load, and it takes more time to change the page when i click any link.

I suggest you start your own thread by asking a question (click here) and including all the relevant information, that way someone may try to resolve your own problem rather than just make general comments.