I tried to install MarcoEditor for Mozilla/Firefox/Thunderbird last night. Note that there is a warning on the support site about problems uninstalling the macro due to old format XPI file. I had no intention of uninstalling it, however, so I went ahead anyhow. Everything went well for Firefox, but when I installed it to Thunderbird something hung up or interrupted the install (I’m not sure which at this point) and so I had an incomplete installation and Thunderbird ceased functioning.
I’m not writing this to complain about the extension. In fact, I intend to revisit it as soon as I have time. I haven’t notified the developer, because it appears to be something he has already noted. What I want to note is the fix. In order to restore Thunderbird to complete functionality, I uninstalled the program and removed all files from the install directory. I did not have to remove the profile directory so nothing was lost. I had made a complete, convenient backup file of all the directories just in case, but everything restored correctly without a hiccup. Similar work was required on Firefox, but again I didn’t have to work on the profiles at all.
This looks like an interesting extension, but heed the warnings on the developer page!