Yes, mine does it occasionally as well, it is a unacknowledged bug in the latest version of flash.
It doesn’t take long to Google and find MANY other people with the same issue (Doesn’t matter if you use Chrome, Firefox or IE).
I was browsing the Adobe forum recently and found a page linking to this. Adobe don’t seem to support people on their own forums, but pay this company and monitor3complaints here!
I don’t know what is causing it as I made a very intensive flash site and the CPU doesn’t break a sweat where as a simple small advert on others causes 100% CPU as you report. When I installed Adblock Plus, it blocked these adverts and the CPU was normal.
It doesn’t help though on some sites where there is needed flash which causes problems. I have no idea what features in flash actually cause this.
*I know it is flash because as a test I downgraded the version and the CPU never went above 30% on some pages then the moment I upgraded to latest, the same page just jumped to 100%
I disabled the process in add-on manager, then quit Firefox and re-enabled it.
This worked, but I don’t know how long it will last. It’s worth a shot though.
I checked my extensions as suggested by PhiLho above and found an “Ask.com toolbar” that I had not knowingly installed. I uninstalled it and the problem is gone.
Guess you also have a couple of Java plugins installed, go to about:plugins page to verify this.
I’d suspect one of them
I had this problem on one computer (and only this one), Googled a lot, found various “fixes”, none really worked…
Upgraded to Firefox 3.5… problem gone.
Guess what, for me it was AVG safesearch in the Extensions tab. Problem Gone. Stupid AVG.
The solution to this issue us simpler than many so-called solutions I have found.
Step 1 – Go to Help/Troubleshooting Information on the Firefox menu
Step 2 – Find and press ‘Reset Firefox’ under the Reset Firefox to Default State area
My CPU usage dropped from over 90% to under 10% immediately and has remained so.
This is necessary on a basis that is determined by individual usage.