All my tests when upgrading to WXT 0.20.x have broken as fakeBrowser is typed as any. Digging into this a bit more, I can see that the FakeBrowser interface uses types from @types/webextension-polyfill but this is listed as a dev dep: https://github.com/aklinker1/webext-core/blob/main/packages/fake-browser/package.json#L55

Installing the @types/webextension-polyfill package directly, resolves the any problem in my WXT project.