Concern with "No need to modify" comments by max32x Rated 5 out of 5 stars
Although the method for making "Open In RegEdit" work with Firefox 4 described by max32x below will work, it will also turn off version checking for ALL other add-ons in Firefox 4. Personally, I'd rather selectively turn off version checking for EACH add-on that doesn't work with the latest version of Firefox so I have some semblance of control. The method I suggested (repeated below), will only allow THIS add-on to work in all versions of Firefox 4.
Simple fix for making "Open In RegEdit" work with Firefox 4.0 and beyond
This has been tested and proven to work with Firefox 4.0 running on Windows XP SP3 32-bit and on Windows 7 SP1 64-bit. This change should allow it to work through Firefox version 5.0.
1. Search for directory (folder) "openinregedit@firefox" (without the quotes). If you find more than one, do the following steps for each one found.
2. Within the "openinregedit@firefox" directory, find file "install.rdf" and open it using Notepad.
3. Change line
4. Close Notepad answering "Yes" to the "Do you want to save the changes?" question.
"Open In RegEdit" will now work properly in Firefox 4.0.