Found out why OneTab wasn't working entirely by accident. Still disappointed by the lack of response from the devs. If anyone else has an issue with OneTab not loading, though, try this:

"I'd changed 2 settings in about:config

image.webp.enabled was set to false (default true)
image.http.accept was set to "*/*" where the default was "image/webp,*/*" - all without the quotes"

I found this for a seemingly unrelated problem with YouTube thumbnails not loading, but sure enough it also corrected my OneTab issue. No idea how the settings were changed in the first place.