Before we shipped Firefox 22.214.171.124, Firefox didn't run on Mac OS X 10.5, A.K.A. Leopard, very well. With Apple's new OS release just around the corner, we put a number of significant Firefox on Leopard improvements into this security and stability release.
But we didn't get everything fixed and there are still a few outstanding problems with Firefox on Leopard that we hope to get fixed soon. Because Firefox 126.96.36.199 included improved, but not 100% support for Leopard, we thought it was good to point out the remaining issues. That was the purpose of the note about Firefox on Leopard "known issues" in our 188.8.131.52 release notes.
We were not announcing new issues here. Nor were we talking about issues that impact OS X 10.4, or any of the supported Windows versions.
The problems with tab closing or Flash and Java plugins are specific to Firefox on Mac OS X 10.5, Leopard, and do not have any impact for the rest of the people upgrading from Firefox 184.108.40.206 to 220.127.116.11.
So, if you read something that made you think you should avoid upgrading to this security release because of new problems, please disregard that information. There are no known new issues in this release and there are lots of great fixes that will make you safer online.