Try the Firefox SafeMode.
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)

1. You can open the Firefox 4/5/6/7 SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
2. Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.
   

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

i just try the solustion here: http://kb.mozillazine.org/Corrupt_localstore.rdf

at first, it works, but after about a minute, i cannot browse the file again :( ok, now i will try yours. thanks for fast response.

See also:

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

I even tried Reinstalling Firefox. mine was worst, i cant even file>open file. import/export bookmarks. practically everythings that requires DIALOG. pls help!

thanks alot