Google seems to consider Pale Moon as the old Firefox, even using native user agent.
Google Docs still seem functional at least, except the first page doesn't show recent docs, console gives invalid state error (might have been broken before, not sure), can open a Word document and Excel spreadsheet, no errors about missing JS stuff.
Saw some talk about polyfilling missing Regex stuff on
palefill extension issues page, they mention XRegExp, a powerful JavaScript regex library, but that it may not be suitable for our use case - using extension to add missing stuff that web pages want.