Anyway the rendering thing is getting worse each day, specially with long pages. Take a look at this example: [asp.net...] (please don't remove the link, it is a well known website) Browse it with Firefox and IE. Scroll down using the bars. It flicks with Firefox.
And this other thing is worse. Firefox is each time less multithreading. I remember when I could take a page and open in new tab each link of that page and all pages loaded in the background without a problem. Now, if I click using "open in new tab", I have to wait until the other page is loaded in the background so I can continue working in the same page.
Many such problems can be cured with a clean install of Firefox, especially if you did not do a clean install at the Firefox/1.0.7 to Firefox/1.5 transition.
Older extensions and themes can also be a problem, especially if you've overridden the compatibility checking by using the Nightly Tester Tools extension.
If you haven't done so, uninstall Firefox, then reinstall it clean. Your bookmarks and preferences will not be lost by doing so, unless something is seriously wrong with your computing environment (You can make a backup of your profile(s) if you're worried about this).
My firefox got slow and crashed constantly a couple of weeks ago. I switched back to explorer for a while and uninstalled firefox.
I reinstalled firefox a few days later, and it has been working nicely ever since. At the time, I thought it might have been "cured" due to the windows updates that had come through the day before I reinstalled.