Firefox Uses 90% CPU when Clicking and Holding

Whenever I click and hold my Powerbook G4's mouse button in Firefox, 
Firefox starts using 90% of my CPU.

For example, if I go to:

http://www.mozilla.org/support/

and then click and hold anywhere on the page so that the context menu 
comes up, WHILE I'm holding, the Acivity Monitor shows that Firefox is 
using 90% of my CPU. Once I let go, the context menu stays up, but 
Firefox drops down to its normal CPU use (.7-.8%).

I have the same problem in Thunderbird, but I do not have this problem 
in any other applications I've tried. It seems to be Mozilla specific.

Does anyone else notice this problem?