Fixes

[ro-ka]

• Fix naming in error messages ('is not exists' -> 'does not exist')
• Fix file names in firefox manifest (they were missing a -)

Note for the tests: Maybe it would be better to generate the compare hash from a 'expected' file so it is easier to spot such things.

[ro-ka]

Thanks, you're incredible fast! :)

[ro-ka]

When will these updates come to npm?