When you're developing a cross-browser add-on, you probably want to try it out for a while in your daily browser. Unfortunately, contrary to Chrome, if you temporarily load your extension in Firefox, it will be gone after a restart.
That's because Firefox needs to sign your add-on before you can install it anywhere.
The documentation explains so, but in a convoluted way. It's actually pretty simple:
Register at FF's Developer Hub (top right) if you don't have a FF account yet.
Go to the add-on submission page.
Choose 'on your own'. This wil immediately sign your add-on, but it won't be listed in the add-on site for distribution. Ideal for a test version.
Upload your zipped add-on files.
Sign, download the
.xpifile & enjoy. It can be installed from
about:addons → gear icon → Install Add-on from file