I have the a similar problem but with the menus, all are slow to respond, also the spell check, if I misspell a word and right click it, it take bit for the spell check to come up. I also tried the Firefox from the Mozilla website with the same results. I noticed its happening, when I'm hooked up to the internet (dailup). While off line Firefox is responsive.
Have you tried opening Konqueror and seeing whether browsing and menus are still slow?
Or, do you have many addons installed in FireFox ? You could try disabling them all and then enable them one by one and see if that helps at all.
Another option would be try opening Firefox with a new clean profile (to do that do: Main Menu > Run Command > and in the box that opens type "firefox -P" (without the quotes and press enter)
That will then bring up a new dialogue box where you can give the new profile a name. Give that a go and see if that speeds thing up at all.
BUT being on dial-up can slow things down, especially on certain sites, and as you say its fine when you're off line
Another option would be to try Google's Chrome browser which is in the repo's (as is Opera)
Thank you menotu and CaptainSarcastic for your response and advice.
I tried Konqueror and no problem with the menu bar at all, same with seamonkey all worked good.
I figured it might be the plugins or add ons, so I downloaded Firefox from the Mozilla site and experienced the same problem. I also deleted and reinstalled Firefox from the repo's and same thing again.
The dailup( I live in a rural area) is a little slow but I don't mind it, it's with the Menu bar (File, Edit, View, History, Bookmarks, Tools, Help) where the problem is. There is a hesitation that is quite annoying. If I have to I can use an alternative browser, but I'm use to Firefox.