It's possible that you are having a problem with some Firefox add-on that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal?

Whenever you have a problem with Firefox, whatever it is, you should make sure it's not caused by one (or more than one) of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in safe mode (don't forget to select Disable all add-ons when you start safe mode). If the problem disappears, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and themes and this one for plugins.

If you need support for one of your add-ons, you'll have to contact its author.

If the problem does not disappear when all add-ons are disabled, please tell me, so we can work from there. Please have no fear of following my instructions to the line, as all can be easily undone.

See if cleaning your "places" database lessens that lag. https://addons.mozilla.org/en-US/firefox/addon/13860/

The other thing that might be contributing to that problem is the "sessionstore" for open Tabs. It hasn't bothered me enough to look for a fix, but maybe someone else will have an idea about that.

I have an Asus EeePC 900A 20G that came with Xandros Linux and have since updated the Ubuntu 10.04 - and I also have access to a EeePC 1000 Win7 Starter, that has the 1.6 duo processor; the 900MHz processor in the 700 & 900's just isn't fast enough for Win7, IMHO.