Unless something changed in the past 3 weeks, you can still upload manifest V2 plugins to chrome, I uploaded one a few weeks ago.

Otherwise, what I did to support v2 for firefox and v3 for chrome is fairly straightforward.

You write your code and compile it all with webpack into a single file, all you have to do is write two manifest files which are 70% the same.

There are some incompatibilities between v2 and v3, like v3 having promises for the extension api. I wrote wrappers for the non-promise versions myself (I'm sure there are packages that do this for you). Webpack takes care of a lot of the other inconsistencies like module support.

Also note that you should 100% make sure that you follow the guidelines for how to structure a plugin for the v3 version, ie that it is event based. Your background scripts will be shut down randomly every now and then and you will get insanely buggy code when running in v3 if you don't follow the guidelines strictly.

For now, I just ship v2 to both firefox and chrome, it's easiest for me. Note though I have had this plugin for a couple years now, maybe it's not possible to create _new_ plugins with v2.