Recently my Firefox 1.0.6 (Windows XP SP2) is having 100% CPU usage for several seconds (to be more exact, 10 to 30 secs on a 3+ GHz machine) on a certain website. Using Sysinternal's Process Explorer, the thread at fault has been located as "firefox.exe!jpeg_fdct_ifast+0x558f". At first I thought it was a browser incomatibility issue of the website which has changed its layout just now.
A friend of mine who is also using Firefox on Windows however did not have that problem, which made me thinking. Then yesterday, he installed Adblock Plus, and suddenly got the particular problem I described to him. So, it's clear now that Adblock Plus is at least partially faulty on this issue.
Please check the website (which is in simplified Chinese) out yourself: