What add-on are you using? Is there some type of information you don't want filled in? Or a website you don't want to autofill?

It's a general privacy concern. The use case is a situation where I allow a guest to use my desktop browser and I'd like the address bar to not autofill.

I suppose one option could be to create a Guest Profile, but this seems like overkill.

Create a separate "guest" account in the Operating System is the best approach to maintaining privacy on your own device.

Agreed. Each account would have its own profile.

Switching profiles, even at browser level, is too cumbersome for my use case eg where I want to let a guest jump on my desktop browser for a few minutes.

I'm looking at possibility of developing an Addon to accomplish this task - essentially a shortcut of (on Mac) Firefox menu>Preferences>Privacy>Location Bar> uncheck/recheck History radio button

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Go to that message, and use the Undo link to its right.