Exactly what I needed and works like a charm. Nice and simple.
The developer also immediately responded to a PR and was very friendly.

better set "browser.zoom.siteSpecific = false"

awesome. is it possible make it work with "open in sidebar"?

This is the only extension that provides that kind of functionality. It saves me a lot time and effort from zooming and resizing content.

An absolute lifesaver when hooking up a HiDPI laptop to a 1080p monitor.

Works for me on Debian Sid. Now my Firefox doesn't look small when I drag it to my HiDPI screen.

I really liked this extension. I also have one use case which should be rather easily to implement. I like using my web browser at 200 percent zoom while at full screen, and at 100 percent while the window fills only half of the screen. Right now the extension supports change of the scaling only when the window is "wider than x pixels". Also is required a trigger for "less than x pixels". Would it be possible to add this feature? Thank you very much.

If you use multiple machines it would be handy to be able to toggle on/off in the toolbar or to set a resolution to toggle from or percentage limit. I use a hidpi on my laptop, but a "moderate" dpi on my desktop and I don't want the zoom on my desktop, but it isn't configurable. Great start though, thanks for the initial go at this.