Anyone find a fix for the flash player issue in 4.2 with Firefox?
You want (need, actually) to be using Firefox BETA (which is available in the play store right next to Firefox for Android, but on a Nexus device you ALWAYS want the beta). Second, you specifically want to be running the "Nov 15, 2012" release of the build, not any previous version. This is, as of now, the only build of Firefox that works at all on the Nexus 7. This build just hit the repositories late last night, so you may need to refresh a few times (go into the detail screen, back out, go back in, lather, rinse, repeat) until you see "Nov 15, 2012" as the release date. Accept no substitutes!
Running this configuration, I'm getting perfectly stable Flash (well, as stable as Flash has ever been) in Firefox on both my 4.2-based Nexus 7 and my 2.3.4-based HTC Thunderbolt.
What issue are you having, specifically?