I have never (to my knowledge) done anything to change the user agent. I dug around in about:config and discovered that general.useragent.override somehow got set.

Edit: Something is resetting that override every time the browser starts. Dunno what, the only addons I run are uBlock and RES (and now TST).

Edit 2: It was being set in user.js. No idea how it got there...

Do you have

  privacy.resistFingerprinting = true

set?

Nope, I discovered that it was being done in user.js. No clue how that happened...

I believe

  privacy.resistFingerprinting = true

spoofs user agent. Does anyone know if there is an addon or to whitelist which pages you don't want it spoofed on?