Flash suddenly stops working in Firefox v3

Sometime in the last week or so, Flash stopped working me on many sites. It seemed to stop working around the time of the last Microsoft Vista updates, but I can't be sure. The strange thing is, it worked on some sites (like Adobe) but wasn't working on a lot of the sites I was visiting. I tried reinstalling Flash, but that didn't fix it. Since it wasn't a high priority for me, I haven't had time to look into it until today.

Today I was doing some QA on our site and noticed one of the Flash components was no longer working. Since Flash seemed to break for me around the time of the last Windows Update, I thought I should probably look into things to see if this is a wide spread issue.

I quickly fired up a couple of different versions of Firefox (and on different PCs) and they were all working fine. This seemed to indicate the problem might be local to my installation.

Next, I decided to actually examine the source code in Firebug—which is when I noticed something odd. The <object /> tag was grayed out like it was hidden or being ignored. So, I fired up the same page in another Firefox install and the <object /> tag was being shown as expected.

NOTE:

At this point I also checked some of the sites I remembered where Flash was working and sure enough they were using the <embed > tag instead of the <object /> tag.

Since I thought this was extremely odd, I decided to fire up a brand new Profile to see if things would work in an empty Profile. Dropping to a command prompt, I used firefox -P to create a brand new profile called "test". Next I used firefox -P test -no-remote to open up the new profile (the -no-remote argument allows you to open a separate instance of Firefox using a different profile.)

When using the new Profile, Flash was worked as I expected. This told me something was wrong with my normal Profile, but what?

In order to track things down, I first tried disabling add-ons that looked like primary suspects (Firebug, Web Developer, AdBlock Plus, etc.) Nothing changed. I next tried uninstalling these add-ons. Flash still not working. I finally just uninstalled all the add-ons. And what do you know? Flash started working again.

In order to try to determine which add-on was causing my Flash problems, I tried installing each add-on individually. Unfortunately (at least from a troubleshooting standpoint) after re-installing all the add-ons Flash is still working correctly.

So I obviously had a problem with my profile that was caused by one of the add-ons I had installed. Unfortunately, I do not know which add-on was the culprit. I do know that after removing all my add-ons and reinstalling them Flash is now working properly again.

UPDATE:

Here are a list of the add-ons I had installed, all of which have been reinstalled:

Adblock Plus 1.1.1
Clear Cache Button 0.9b
ColorZilla 2.0.2
Download Statusbar 0.9.6.5
Firebug 1.4.3
Firesizer 0.92
Free Download Manager 1.3.4
Html Validator 0.8.5.8
IE View 1.4.4
Microsoft .NET Framework Assistant 1.1
Organize Status Bar 0.6.3
Page Speed 1.3
Print Context Menu 1.2
QuickFrame 0.5.1
QuickProxy 2009.07.19
Web Developer 1.1.8
XMarks 3.3.2  (NOTE: I didn't actually uninstall this add-on, it was the one exception of an add-on that I did not uninstall.)