I think the main culprit is Google Toolbar.     So uninstall it and see if it makes a difference.

Another reason is fragmentation in the Firefox internal database.   There is an add-on that “cleans” the Database.

addons.mozilla.org/en-US/firefox/addon/13878

Please post a comment if this helped you.   Thanks.

EDIT:

Also try this tool   memoryfox.blogspot.com

you have to have a sandbox account to be able to install it.