For many months a buglet had been niggling at me. Anytime I opened an XML document in Firefox the entire browser would freeze for several seconds. If I was trying to debug a RSS feed or an XML settings file this would quickly become extremely annoying.
Today I fixed it.
First I tried a few of the obvious things. I already knew that this was not a core Firefox bug because this did not happen on any other PCs – it had to be something related to my PC. I checked the default profile with all addons disabled – still froze. I tried using a profile in another user’s account – still froze.
Then I had an idea how to find the culprit: I fired up regmon. After a few tests I quickly narrowed it that the freeze was happening right after loading the key HKCR\feed\shell\open\command\(Default)
I checked the contents of that key and I saw immediately what the problem was. The key was pointing to a program on a shared folder – so every time Firefox loaded XML it would attempt to mount this shared folder and this was the cause of the freeze.
After deleting the registry key my Firefox is back to being speedier than ever!