This is most likely being caused by the ClearURLs addon. It's been a longstanding silent problem of the addon. It just breaks a lot of things on Google Sheets. A fix is supposedly being worked on now.

Look in your Addons area, disable that addon, restart Firefox and test searching on Google Sheets again. If you don't have that specific addon, try disabling other addons to see if they might be causing the issue as well.

Read this article for disabling addons: Disable or remove Add-ons

This fixed it, thank you! I like that this add-on prevents tracking so I'd love to find an add-on that still does that but doesn't break Sheets. Do you know of one?

You're welcome! It took me a while to remember but Neat URL is the other addon that attempts to strip out the tracking parameters.

Neat URL: https://addons.mozilla.org/en-US/firefox/addon/neat-url/

Just be on the lookout after installing it. It hasn't been updated since 2020 but most urls with tracking links haven't changed much. So it should still clean the majority of links. But I would just add to always keep in mind, if something breaks on a website and nothing you do seems to fix it, you might need to disable this addon as well to make sure it's not doing the same thing as CleanURLs. So far after reading its reviews, I'm not seeing reports of site breakages. Take a look yourself: https://addons.mozilla.org/en-US/firefox/addon/neat-url/reviews/