Actually, modern extensions are rather similar between browsers, thanks to the WebExtensions API that was created a while ago. Mozilla even has a porting guide and list of differences: https://extensionworkshop.com/documentation/develop/porting-a-google-chrome-extension/

However, they still need to distribute different packages, at least for the time being. But I can see a hypothetical unified store just letting you pick between the “Mozilla” or “Chromium” versions of extensions (perhaps automatically, based on your browser).