- webextensions specific storage apis to return promises but chrome still needs callbacks
- A few UI differences between chrome's extension popup and firefox's one means you'll need to potentially leave out features for one browser.
- csp policies differs between chrome and firefox(and cors too I think)
- UX differences between browsers means you'll need to write extra code, and maybe a few extra tutorials.
- Difference between how permissions are interpreted on different browsers
- Huge huge difference between publishing on chrome vs on firefox
- Safari requires xcode, and therefore macos to publish