I have the exact same problem. My memory is not full (maybe 500 Mo out of 4GB free), firefox taking about 1Go. CPU usage fluctuates at normal levels except some peaks at 90 or 100% for no good reason.

Sad thing is, this has forced me to start using Chrome for the moment. It's not just the subject-field, other things as well causes FF to use 90-100%. But the subject-field is the most easily reproducible one.

/ A/

well, I still stick to FF.. are we the only two having this FF slowness on mac osx? is this only occurring on 10.6, and has every one switched to 10.7 or 10.8 ?

Does it help if you disable form auto complete?

• Firefox > Preferences > Privacy > History: "Remember search and form history"
• https://support.mozilla.org/kb/Form+autocomplete

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Hey, thanks! It worked fine in safe mode, so I turned off hardware acceleration and now it works fine.

Thank you so much! I just restarted it in SAFE, disabling that history-thing, and restarting normally again. And now it's fast again! Can't say what part helped, but the above steps as a total sure did.

Also, as a side-effect, I noticed how fast FF is when everything is switched off. Man, that was a bummer to see (since I actually use my addons, all are mandatory).

Thanks again!

/ A

Disabling a number of extensions improved this, but it was solved when I both unchecked the "Remember search and form history" option and disabled hardware acceleration.

Re-enabling hardware acceleration brought the slowness back. Hope it's not helping for anything else!