I have Firefox 16.0.1, and as of lately, it's become too unbearably slow for me to continue using it. Even when typing this message, at certain points it will lag. When switching to new tabs or clicking links, it lags HORRIBLY. At least 4-5 seconds of wait time. And now it's started to screw up my enjoyment of YouTube videos by lagging when scrolling down the page (as it does with other sites) and lagging the video itself. I've tried the reset feature. That did absolutely nothing for me.
The only add-ons that I have are AdBlock Plus 2.1.2 and the FTDeepDark 4.8.3 theme. And I've already tried disabling (even removing) them to no avail.
What must I do for Firefox to run properly and quickly? I've been a huge fan up until now, and I'm seriously fed up with it. I have a very quick laptop (Win7, i7920XM, 8 gigs of DDR3, 256GB SSD, dual 5870 GPUs, etc.), and Firefox has always run great up until the past few month.s
Try the Firefox SafeMode to see how it works there.
When in Safe Mode...
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)
Don't select anything right now, just use "Continue in SafeMode."
To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.
Try to disable hardware acceleration in Firefox.
Running in Safe Mode resulted in less lag at the beginning, but over the course of a few minutes of browser use, it started lagging more and more.
Running with Hardware Acceleration disabled seemed like it was working somewhat at first, but soon went back to lagging just as bad as before.
Create a new profile as a test to check if your current profile is causing the problems.
See "Creating a profile":
There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.
If the new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.
I'm also having lag problems with the latest version of Firefox ... I'm running XP with a second monitor (primary monitor is a laptop screen). My issue is this: when I open Firefox in the second monitor, it (i.e., the cursor) lags badly, especially when mousing-over links on web pages. However, Chrome works very well on the same monitor and both Chrome and Firefox work fine on the laptop screen. This is a bizarre problem ... not sure what I can do about it. (It's bad enough that I think I will at least temporarily switch to Chrome.)
I am also having this issue with the latest version (18.0.1). I do have a fair amount of tabs up, but much less than usual and never had an issue. There is no issue with flash (I have nothing with flash up in the first place). But for some reason, it starts just lagging like crazy out of nowhere. I have tried one of the solutions given through the support pages that says to rename one of the files--while this works...it only works temporarily. It's not long until it starts lagging ridiculously bad again.
It isn't even anything with my computer. Other browsers are fine, games are fine, literally the ONLY thing lagging for some reason is firefox.
Firefox 20 has been doing this for me too. After a while the browser gets extremely laggy. Clearing out ALL History doesn't work. The only thing that does is to quite the browser and relaunch. Pretty inconvenient when you have to restart the app every few hours, especially when you have downloads in progress. When this happens, I usually just switch to Safari or Chrome to browse the internet. I may just scrap Firefox all together and stick with Chrome. No issues with Chrome (or Safari). So I know it's not my Mac Pro. It's the browser. I'm guessing it some sort of memory fragmentation with the browser.