Fortunately, there are at least three options which can fix Firefox’s lethargic performance.
1. Clear the History
Older versions of Firefox allowed you to set the number of days Firefox retained browsing history. The option has been removed in version 6 and your history will be retained indefinitely (there are about:config options, but it’s not clear how these impact the size or efficiency of the data set).
For me, clearing the history had the most dramatic impact on speed. Click the menu > History > Clear Recent History (or Ctrl + Shift + Delete on Windows):
Choose “Everything” in the time range, then ensure “Browsing & Download History” is checked. Optionally, you could select every option if your speed issues aren’t rectified. Click “Clear Now” and, once it’s complete, restart the browser.
If the speed issues re-occur, you can configure Firefox to clear the history when it’s closed. Select the Privacy tab in the Options dialog and check “Clear history when Firefox closes”. The Settings button allows to you specify which data is cleared:
However, be warned that automatically clearing the history also wipes Panorama’s tab groups. Let’s hope Mozilla address that soon…
2. Clear the Cache
The latest versions of Firefox implement automatic cache management which should, in theory, use the best settings for your PC. I’m not convinced. On my installation, the cache was approaching 1GB and it was probably taking Firefox longer to locate a file than downloading it again.
To fix the cache, open the Options dialog, click the Advanced tab, then Network:
Hit “Clear Now” and make yourself a coffee. It may take several minutes and Windows installations often report that the application has become unresponsive. Don’t worry and let it complete.
Now, check “Override automatic cache management” and limit the cache to 50MB.
3. Disable Hardware Acceleration
This may be counter-intuitive, but many users are reporting that hardware acceleration can have a negative impact on Firefox’s performance. The fix didn’t noticeably affect my installation so perhaps it depends on your OS, graphics card, drivers or other factors.
To change hardware acceleration settings, choose the Advanced tab in the Options dialog, followed by the General tab. Then uncheck “Use hardware acceleration when available”:
Restart Firefox and check the speed. You can re-enable acceleration if there’s no discernible difference.
If All Else Fails…
If speed remains problematic, you may need to create a new profile or completely uninstall Firefox then install again.
Or switch to Chrome.
I hope these tips help you. My installation has become usable again.