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:

1. Register at FF's Developer Hub (top right) if you don't have a FF account yet.

2. Go to the add-on submission page.

3. 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.

4. Upload your zipped add-on files.

5. Sign, download the .xpi file & enjoy. It can be installed from about:addons → gear icon → Install Add-on from file