I installed it last night and am happy to report without any issues. Everything works, and all pluggins (PDF, Java, Real alt, QT alt, Flash) and extentions (although I only have Adlock and the default DOM currently installed) are functional without any special steps having been taken on my part....I even forgot to disable adblock before I upgraded (cause I forgot its not listed in the Tools>extentions, but rather is a unique entry itself i.e. Tools>Adblock) and no reprocussions ensued.
The icons look nicer too.
In fact, I'm much happier with .9.1. When I had previously upgraded to the initial .9 release, aside from all the highly annoying installation issues I experienced, I also felt that the browser was discernably slower in every day use compared to the former v.8. I don't know if that degradation in performance was the result of the troublesome v.9 installation and was thus limited to me personally, or whether the problem extended to every user. Regardless, this newest release has, for me personally, restored FF's prior snappiness. So as to what percisely this is attributable is anyone's guess.