I'm running Ubuntu MATE and whenever I try to torify something, it runs in the background but doesn't show a GUI (Firefox, Thunderbird, etc)
torsocks (torify, usewithtor, tsocks, et al) isn't generally suitable for large, complex applications (or specific types of simple application). To ensure connections are made through tor and to help prevent against leaks, it hooks multiple libc function calls. Some of them are even disabled if they cannot be safely torified. As such certain functionality that applications might normally expect to be available won't be and it may end up breaking it in unexpected ways.
It should not be the first method used to try to connect an application over Tor, native SOCKS5h or SOCKS4a support should be preferred. In this case both Firefox and Thunderbird already have native SOCKS5h support, however they may end up leaking which is why you should prefer using Tor Browser, which has specific patches to prevent this, or an operating system with a desktop environment configured to work within a tor-enforcing like Tails.
Try to install FoxyProxy Extension.
I had the same problem when using the
torify command. The only way to solve that was the extension.