Layer Keys by plucafs

The shortcut is activated pressing CTRL+Comma in a non-system page (all the pages except the firefox native one, like settings, new tab, etc.)
Clicking on the addon icon will show a popup with a button to open the settings. There, you can add layers and actions.

How to change the default shortcut (CTRL+Comma):
1. Open about:addons (paste it in the address bar). It'll open the Firefox's settings page, specifically in the Extensions page
2. Click on the little gear on the top-right and select Manage Extension Shortcuts. It'll open a page listing all the extensions that have defined at least one custom shortcut
3. Look for the title Layer Keys and edit the default shortcut