*UPDATE*Looks like after I spent all that time making this entry, that mozilla already had a knowledgebase article up about it. You can find it here:
So, in Vista with the latest Firefox update, I've noticed a problem when opening URLs in applications like Outlook or even the Windows Sidebar. I'll get a popup like this:
Opening a link in Outlook makes it look like this:
Either way, the path to fixing it is the same. Open up regedit and go to the following registry key:
Double click on the Default string, which should have a value of ""%1",,0,0,,,,". Delete that value and leave it blank.
That's it. You should not get the pop-up errors anymore. However, since this happened once, it will likely happen again. I believe it occurs when either Windows or Firefox (not sure which) resets the default browser on your computer to be Firefox. So next time that happens as part of a Firefox update or you messing with the Default Programs control panel, you may get the pop up errors again and have to redo this.
To make it easier you can just make a regedit file. Make a new text file, name it something like "FixFirefoxURLErrors.reg" and put the following into it:
Windows Registry Editor Version 5.00
Then just double click on the file and it'll make the registry change for you.
Again, these steps are for Vista only. You may remember running into a similar error to this with Outlook and Firefox a long time ago, it seems to have come back. The steps one would follow on Windows XP don't work on Vista, since Outlook is looking at a different registry key. All I've done out is which registry key that is (thank you Sysinternals Process Monitor program!) and applied the Windows XP fix seen here: http://www.hanselman.com/blog/FixQuotGeneralFailurequotWhileLaunchingFireFoxURLsFromOutlook.aspx
By the way, just a fair warning, you can do a lot of damage in the registry if you delete something you're not supposed to, so be careful, I'm not going to help you if you break your computer. :)