Mozilla released the first beta version of Firefox 4, for public review. Version 4 of the browser is due out towards the end of the year, but you can try out the beta now by grabbing it from the nightly build page. I’ve been using it for a few hours, and I do believe it will give Chrome a run for its money in the speed department. This sucker is fast!
I ran into this problem where I was unable to install Firebug in Firefox. It would act like it was installing, Firefox would restart, but the plugin was simply not there.
A little research revealed there could be a variety of reasons for this, but for me it turned out Avast! antivirus was preventing it from working.
I disabled the Network service in Avast! long enough to try again, and it all worked.
Hopefully this will help someone else who runs into the problem.