[@BingTheWing] No. Though the image will max out at 100% post width.

Ideally the Guild would have a nice drag-and-drop image solution that rehosts it and lets you resize it.

[@Grimhildr] Does that mean you can replicate the issue in a fresh Firefox profile window / incognito window (whichever way you can create a window without add-ons)?

If not, then another possibility is that, since Firefox add-ons auto-update, an older add-on could be causing problems now that it wasn't in the past.