I've got this exact same issue right now with FF23, even did a fresh install of Windows just now but even that didn't help. I reset firefox -- doesn't work. I uninstall & reinstall firefox (3 times now) -- doesn't work. I uninstall & reinstall both flash & firefox -- doesn't work. I uninstall flash for just firefox & install system wide flash -- doesn't work. Yes they are all the most recent versions of flash.

It's really starting to get under my skin. I like Chrome but not enough to use it as default. I really hope there's a solution for this out there. I'm off to try the 24 beta...

UPDATE

• Note this subject continues in
  Flash Forcing Firefox Freezes
  /questions/969468

Just some general notes

1. Uninstalling and Reinstalling Firefox rarely fixes anything
   • Read the article first. (You lose information and may wish to bookmark open tabs and the homesites of any Firefox Extensions.)
     Refresh Firefox - reset add-ons and settings
     You can try that to fix problems,but it is not particularly suitable for FlashPlayer issues
2. Start your own thread use /questions/newfor your own problem.
   Please try to follow the prompts to include all troubleshooting information.

• edit use this direct quick special LINK

1. • if you consider it strongly related to this thread post in the thread to confirm you have started a new thread.
2. Some of you may have mentioned newer versions of Firefox work with Flash. Note the Aurora channel has a new and improved reporter for Flash problems. (Not sure it is ready yet to move up to Beta and Release it is only on trial at present). I am keeping fingerers crossed that the new reporter will help the developers when they need to look at our Flash problems where there may be a hang or a crash..
   • http://www.mozilla.org/en-US/firefox/aurora/
     and beta Firefox 24 was just mentioned http://www.mozilla.org/en-US/firefox/channel/#beta/beta-desktop

I had this problem for a very long time - first on Windows 7 (32-bit) and then on Windows 8 (32-bit). I disabled Flash protected mode (just like John99 suggested). That didn't help. Then I disabled hardware acceleration for Firefox and for Flash. This didn't solve that problem. However, it made Firefox much more fluid and RESOLVED the issue of distorted icons in my browser, which had been a problem for several latest versions of Firefox as well.

At last, I also completely uninstalled Microsoft Silverlight from my computer (only disabling the plug-in within the browser didn't help). FinaIly, this RESOLVED the problem of freezing and unresponsiveness on the browser start. My Firefox now works like a charm after a long time again.

There certainly are some threads mentioning problems with Silverlight, and posts to say Flash and Silverlight together cause problems.

We do not know how many posters in this thread have Silverlight and Flash or if they find the combination causes problems. Readers of the thread may of course vote for helpful posts so those seeing problms with Flash Player resolved by uninsatlling Silverlight may wish to vote for leon.slo's post above.

Thanks for the reply, but, I've never had SIlverlight installed.