Alex Keybl, engineering project manager at Mozilla offers up an explaination over on Bugzilla:
For everyone following along, we built Firefox 9.0.1 with bug 708572 backed out (see https://bugzilla.mozilla.org/show_bug.cgi?id=708572#c11). We've pushed Firefox 9.0.1 for all platforms. Although we think Windows is mostly unaffected, we still decided to move forward with Windows->9.0.1 for the following reasons
* We were living with 702813 for most of the life of Firefox 9, and that the backout in 708572 has actually gotten less testing * New crash signatures (Windows even) have been found which are related to 711794, although none yet are significant or startup crashers * We are uncertain about the number of sites affected by 702813 * We are uncertain about whether or not we 100% fixed 702813 with the backout in 708572 * The population of Windows users who have already updated to 9.0 is small (in the low millions) * There is tracking/analysis benefit of having all desktop platforms on the same Firefox version
The bug fix that's been removed from 9.0.1 was causing problems when certain toolbars were installed
Some Firefox users were so freaked out by the sudden and unexpected reelase of 9.0.1 that they thought it was a hoax:
If you've already got Firefox 9.0 installed it is recommended that you download this update.