Someone has a thread discussing this as a known problem with 0.5.8, or 0.5.8+ (I forget if one or the other or both) being known to break the nightly window and extension and menus operations in Firefox 1.0+
-- I found it discussed on a web page somewhere, a few days ago, finally.
Where the heck? Rats, I thought it was here.
Anyhow it's a known problem. To someone.
I'm using OSX nightlies of Firefox, and have been safe using AdBlockPlus "Icon" 0.5.7+ with them up through Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8b4) Gecko/20050717 Firefox/1.0+
Note, I do regularly create new plain vanilla Profiles and move over my bookmarks, etc. and reinstall each extension by hand, when updating to a new nightly produces any odd behavior at all in any veature.