Follow-up from "Fix mistakes + Add firefox-esr test on CI"

The following discussions from !13 (merged) should be addressed:

• @sfargier started a discussion: (+2 comments)

  Should we have separate jobs (test-firefox-esr) ?

  This should provide the following:

  • tests are still "fast"
  • tells if it's a browser issue or a test issue (if it fails everywhere)

  Are you also sure that it's a version "<65" ?

  can you indicate the Jira or gitlab ticket in .gitlab-ci.yml file comments ?

• @sfargier started a discussion:

  We should find a way to test on different browsers not duplicating this rule.

  Maybe we can move to an environment variable (BROWSERS, or KARMA_BROWSERS), look for it in karma.conf.js and set it in the different .gitlab-ci.yml jobs ?