also, i was looking for a setting for a mail to juscool and ran across "browser.xul.error_pages.enabled" in the firefox "about:config" page -- this browser sleighs (sic)! this setting makes life for tabbed browser junkies easier -- instead of errors producing those annoying dialogue boxes, they will just load as an error page in the targeted tab.
other firefox extensions i couldn't live without:
Deepest Sender - post to lj
Mouse Gestures - execute common commands with mouse movements
Single Window - traps links that would normally open in a new window and opens them in a new tab
Bookmarks Synchronizer - synchro bookmarks to ftp or webdav share
oh yeah, and try "about:cache", "about:plugins", and best of all, "about:mozilla"