In order to test, you need to:
Open the new profile and then open this URL. IMPORTANT: You need to be logged in into GitHub in order to be able to complete the next steps.
Click on the first item in the table, on the Builds text right to the green checkmark.
At the bottom of the next page, click on the build and download the file.
Decompress the file and drag the nightly.xpi file over the open Nightly window. This should prompt you to install the add-on.
If it fails because the add-on could not be verified, double check the signature setting in about:config (see beginning of the doc).
It’s important to note that unsigned add-ons cannot be installed in release, you need to use Nightly.
The add-on installed in this way won’t update automatically. If you make changes, you’ll need to remove it and repeat the process.
As usual, Pontoon commits translation updates every 10 minutes to the repository. The next build will be created automatically as soon as there’s a change in the main code repository, but there’s at least one build created every day.
After installation, the add-on will appear in the toolbar.