I've unzipped the xpi files and edited the version number, but I couldn't figure out how to rezip the file so Firefox would accept it as an extension. I have WinZip in Virtual PC and 7-zip. Could someone explain how to rezip the folder into an xpi after editing?
Okay, this is where it gets a bit tricky. It might've been a better idea to extract the XPI contents to a new folder, but we'll go with what you have. Locate the freshly unzipped files, change the version number in the XML file, then right-click and create a new compressed folder (or use 7-zip to create a new archive, file-roller is suitable for the task on Linux). Just drag-and-drop the files to the new archive. When you're done, save the archive, then rename it from .zip to .xpi and it should be good to go.