Maybe a bit tangential, but I have been having a difficult time using Google Sheets lately in Firefox (like starting about 2-3 months ago). I keep track of my expenses (like groceries) in Google Sheets and always use Firefox to do data entry. Nowadays, Firefox browser freezes for a minute or two when I load/modify cells in Google Sheets.
I only have one extension, 'muBlockOrigin', installed in Firefox and have been using that for many years, so I'm sure the extension is not getting in the way of loading Google Sheets. So that leads me to believe that Google is probably supporting less and less of Firefox.
Google products (Sheets/Analytics/Gmail) simply run slower on FF these days. It's clear that Google optimizes their products to run well on Chrome, and doesn't bother benching performance on FF. I have a chromium portable laying around for the sole purpose of logging into google products that I need to access for work, and access everything else in FF.
I don't think Google is maliciously making FF slower on their products, but I believe they simply don't care to tweak/tune performance on FF like they would have years ago. As everything else google does, the devs metaphorically cover their ears and proclaim "lalalalalala we are google we don't care!" ...and it's more telling every passing year.
Create a fresh profile and install nothing into it and don’t change any defaults at all. If the problem still occurs, open a Webcompat issue about it. If the problem goes away, it’s either the addon or some sort of non-default config settings.
quickest fix if the top of my head would be to stop using google sheets :D
if you only need basic functionality then libreoffice would be a good place to start since it's cross platform and if youre running Windows then there's a portable version that you can bring around on a usb.
I use syncthing myself to sync my documents between all my devices. but there lots of other options around like resilio sync, nextcloud etc there's not really much of a reason to be tied to Google anymore
I only have one extension, 'muBlockOrigin', installed in Firefox and have been using that for many years, so I'm sure the extension is not getting in the way of loading Google Sheets. So that leads me to believe that Google is probably supporting less and less of Firefox.