(Specifically: it now mandates "Manifest V3" for new extensions, which Firefox is still implementing (it basically has to follow what Chrome is doing, given Chrome's reach). Once Manifest V3 works on more browsers, I should be able to relatively easily upgrade, and then support Chrome as well.)
Edit: oh, I should probably also add that I'm a Mozilla employee, but this is a personal project, and I'd have done the same if I hadn't been.
I don't see any breaking changes here.
I wasn't clear enough in my comment, I was suggesting you can make a branch in your project repository for chrome. Make the change only there until Firefox supports MV3.
Having everything be Chrome-first (and, often, Chrome-only) is how we got here in the first place. <3 to the OP for focusing on Firefox first, and putting a pebble on the right side of the scale!