try to update your firefox 6.0.1 to 6.0.2 which is latest

https://support.mozilla.com/en-US/kb/Firefox%20hangs
See hang-at-exit

This may not be too relevant but note Firefox still has a bug (#633427) that results in plugin containers being opened for all plugins, even disabled ones when history/cookies are cleared. If your firefox is set to clear history on closing you will suddenly unexpectedly see some plugin container instances opening up, but they should close down again. (it appears you have about 20 plugins installed - I do not know if you have others that are not enabled)

If that is the problem maybe considering an alternative profile with fewer plugins would be a work around until the bug is fixed.

Thank you!

Thanks. Closing with File/Exit instead of the X closes Firefox properly. AT LAST! A solution!

I'm not sure if that's true. It seems like I did use File/Exit to close Firefox. Was this bug fixed in version 7?

The bug about plugin container opening multiple copies is not resolved as far as I am aware. The problem should only occur if Cookies or History are being cleared.

I have recently seen this problem occur in Firefox 10.0.a1 (Nightly).

There are currently other known issues that may cause Firefox to hang or freeze, some of which relate to the use of the places.sqlite database that holds bookmarks.

• see http://blog.bonardo.net/2011/09/30/is-your-firefor-freezing-at-regular-intervals