Automatically run filters on all folders in Thunderbird

Thunderbird only automatically runs filters on the inbox folder by default. But there is a little trick which will allow you to automatically run filters on all folders and subfolders, including the junk folder, the spam folder and other folders.

1. Open the config editor. If you don't know how do do it, here's an instruction.
2. Create a new string setting with the value mail.server.default.applyIncomingFilters and set it to true. Even if it seems odd, do not create a boolean setting, it has to be a string setting with the value true.

Now Thunderbird should run your filters on all new mails, regardless which folder they're in.