GaryK - 3:18 am on Nov 21, 2012 (gmt 0)
The upgrade to 17.0 rendered my Firefox unusable in many ways.
In particular Manage Add-Ons didn't work; it wouldn't even open. Clicking links caused two new tabs to open and load the linked-to page in both tabs. I no longer got a warning when trying to close Firefox with multiple tabs open. I could only view streaming Netflix & Amazon in 480p even when a HD version was available.
Thankfully I've learned over the years to keep the previous version around until I'm satisfied the new version works properly, so I was able to downgrade to 16.0.2 and now everything that was broken works again.
I think the best bet these days is not to upgrade to any .0 release, opting instead to wait until at least a .0.1 version is available.