Thanks for the update. I was able to reproduce the problem by adding some additional stress. Before doing anything, I added these steps:
• Visit a web page in Firefox 4
• Quit Firefox 4
• Zip the Firefox 4 application package so that it can no longer run.
Now when I click the bookmark which is designated
Visit with Firefox in BookMacster's status menulet, it gets visited in Safari, and Firefox 3 launches but does not show the desired page, as you reported.
However, if I then quit Firefox 3 and click the bookmark in BookMacster again, this time Firefox 3 launches and visits the bookmark as desired.
The fact that the bookmark gets visited in Safari the first time, although not ideal behavior by Mac OS X, is understandable because its previously-known app for org.mozilla.firefox was no longer available, so it had to search for another one.
I'm sure you didn't put it under this additional stress, but maybe with this clue we can figure it out someday. Of course, the problem might go away when you decide to bite the bullet and give up on those 32-bit plugins or whatever is preventing you from upgrading to Firefox 4. Firefox 4 has been available to developers for a couple years, and if a developer has not updated their stuff by now, users should no longer be holding their breaths