+++ I think it's something to do with the roboto font, because removint it from the font family on the page after pressing F12 fixes it, but it works fine in other browsers..

Could it be you have versions of Roboto installed locally, these would take priority over the remote ones, but the local ones have trouble loading? Could you try removing any local Roboto fonts to see if it improves? (Or, generally, clean up duplicate instances or remove all older local occurrences of that font and upgrade to current versions if you need them on your local system?)

There isn't away to remove it in the fonts page in Settings and it doesn't even appear in the Windows/Fonts folder. One time I restarted my PC and it wasn't anywhere, which allowed the pages to load correctly, but now it's broken again.

Just seems to be random, sometimes the font displays correctly and other times it doesn't. Kinda makes the program difficult to use.

FWIW this is a local font issue. I just checked the camel3x site since I didn't see any Roboto there, and, alas, it has a system(~ish) font stack, so for me it's actually using San Francisco — it does not serve any own webfonts, only defers to what the underlying users' OS provide.

Roboto is there on the top of the stack. But they don't serve it. So if it gets picked up for you, then your OS must have it available and provide it to the browser.

If you can't find any corrupt or duplicate font files, try installing a fresh official Roboto type package over it, in hopes it fixes the setup for you.

Well it doesn't happen on any other browser when accessing the same sites so there must be something Firefox is doing wrong specifically. The text on google reviews is also affected. Just turned my PC off and on and it didn't happen this time after booting. I did try manually downloading the font (when I searched it didn't appear on the system already) so hopefully that fixes it.

Since "any other browser" is Chrome–based, and Roboto is a Google Material default font, it might be coming with its own internal version. Or resolves differently. Or to different location.

I'm not familiar with Windows font issues troubleshooting (similar woes were fixed for Mac users by resolving font manager scan errors, from corrupted, badly loaded and/or duplicate font assets), so you'd need to wait for others to chime in regarding good ivestigation steps for Windows specifically.