Most extensions could likely be built and tested just fine on a ~$250-$350 used Mac mini. That's probably still more money than should need to be spent, but a brand new machine probably isn't necessary in this particular case.
I was thinking on that. You still need peripherals if you're using a Mac Mini, though that's no different than if you were using another desktop. Still though, if you don't use, or don't want to use a Mac as your daily driver, that's a dedicated machine just to build/publish an extension for a browser. IMO, there's a slightly stronger case for iOS apps, but requiring that for a browser extension is much harder to justify.
However, if you're the creator of a Firefox/Chrome extension, and want to utilize the WebExtension support in Safari, your deployment workflow can no longer be platform agnostic.
I've done this to build for MacOS, and it still becomes like a tax as the hardware ages out and Apple insists you have the latest OS to publish to their stores.