Fix Firefox max version to 69

[linuxwolf]

This change, combined with an update to the updates manifest, defers to the built-in functionality in Firefox 70.

See BMO 1560608

Connected to #188 Connected to mozilla-lockwise/mozilla-lockwise.github.io#138

Testing and Review Notes

This add-on should no longer install on Firefox versions 70 or higher.

To Do

• [ ] double check the original issue to confirm it is fully satisfied
• [ ] add testing notes and screenshots in PR description to help guide reviewers
• [ ] make sure CI builds are passing (e.g.: fix lint and other errors)

[msujaws]

Any update here? @linuxwolf

[linuxwolf]

At least from local testing, this is doing what's asked, so merging. Will then stage a releas with the latest updates from Pontoon.