"Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If you keep a very large history then try to clean up the history and remove unneeded entries.

You can also test if it works faster in Private Browsing mode.

I have tried that, no changes. Always running Private Browsing mode. However the frequency has decreased now when that error occurs.

I've been having this problem lately as well. I'm running Firefox 3.6.10 on Mac OSX Leopard Final PPC on a heavily upgraded PowerMac 1.8GHz 7448 G4 with 1.5GB of memory and an ATI Radeon 9800 Pro graphics card. This normally flies with both Firefox and Safari.

Until recently, Firefox and Safari ran like lightning on it. Much more recently, this problem has been occurring (guessing since the last upgrade to the latest version), but it's not just closing a tab that can cause a delay. I get shorter delays when clicking on message boxes (usually a second or two, but very noticeable) and even scrolling and typing (it just delayed about 2.5 seconds on the word "and" in this very sentence). Activity Monitor tells me it is Firefox that is causing the problem as the CPU activity suddenly spikes from <20% for Firefox to >85% when the delay occurs each time. Turning off iTunes in the background seemed to decrease the tab close time from 5+ seconds to around 3.5 seconds (beach ball appears either way) and I assume this is due to slightly more CPU time in general being available.

After much testing off disabling add-ons, etc. between writing that paragraph and now I think I've got a pretty good idea what caused the problem. It wasn't an add-on per se (at least not directly). I've had Firefox running for WEEKS on end. On the Mac, shutting a window leaves Firefox running in the background unless you tell it to quit. Well, it comes up faster if I leave it running and I use this machine to power my whole house audio/video system through iTunes/AppleTV and so it's on 24/7. Since the fix in the last update to iTunes 10 (10.0.1), the system is back to being incredibly stable once again (there was a bug in 10.0 and possibly lesser ones in 9.x). I literally have not had to reboot in weeks and thus Firefox has been running for weeks.

When you disable an add-on, it restarts Firefox and this would also clear any memory thrashing or leaks that might have developed over that time (add-ons could be to blame over long lengths of time here too, so I cannot say for certain it is entirely Firefox's fault, but it would seem there is some kind of memory leak or related issue occurring over it running for long lengths of time. I thoroughly tested all the add-ons I use and eventually turned them all back on when none appeared to cause the problem. The problem did not reappear so the only explanation is that it was related to Firefox running for long periods of time. Unfortunately, I do not know exactly how long or how much use was involved to get it slow down to that extreme, but the problem has been going on for days now as I did not quit it. I kind of thought it was some other stuff I was doing in the background, but obviously not. I think the huge delays may have been a transfer from real memory to virtual memory and back of whatever memory was being clogged up. I did not think to look at the memory pool for a big change before I quit so I cannot say how much memory was actually in use at the time, but I do think the problem is not a given plugin, but rather a long term leak. I was not using Private Browsing today, but I was using it a couple of days ago on and off. So whether that is somehow related to the problem starting, I can't say for certain at this point in time, without more testing.