I saw a thread awhile back about how Firefox will log you out when you close the window. Well my firefox randomly started doing it so i found a solution that should work for the rest of you, i tested it on multiple machines with the same issue.
Make sure firefox is closed before doing this. If you are unsure of what you are doing, backup your files. All in all a very simple fix. Wasnt sure if anyone found a solution yet so i thought id post one up.
Here is how to fix it:(note i did it on XP Pro but same basic steps for any version of Windows, i have not tried it on Mac or Linux).
Step 1: Start>Run
Step 2: In that box type "%APPDATA%\Mozilla\Firefox\Profiles\" without the quotes.
Step 3: There will be a Folder called "cuembydn.default" without the quotes.
Step 4: Open that folder and look for a file called "cookies.sqlite" without the quotes and delete it.
Step 5: Enjoy Firefox like you used to!
