Step 1. Fetch and check out the branch for this merge request

git fetch origin
git checkout -b firefox-version-up origin/firefox-version-up

Step 2. Review the changes locally

Step 3. Merge the branch and fix any conflicts that come up

git fetch origin
git checkout origin/master
git merge --no-ff firefox-version-up

Step 4. Push the result of the merge to GitLab

git push origin master

Note that pushing to GitLab requires write access to this repository.

Tip: You can also checkout merge requests locally by following these guidelines.