It allows to install incompatible addon. You can do it though the about:config, but this extension is genious. Why is it genious? Because Firefox keep changing the about:config parameter each time it updates, and ask to enter the current version, so no more headache with this.
While this extension may do what it does just fine, Mozilla has changed their code to require plug-ins to be "signed" before they can work. They will be marked as something that means the original writer never got the xpi registered and approved by mozilla. Recommendation is to switch to Palemoon or another fork of Firefox.
I installed this to firefox to try & stop Fast Dial being disabled by the incompatibility check (even though Fast Dial works flawlessly). Unfortunately this AddOn does not work & Fast Dial is still being disabled every time I reload firefox no matter how many times I reinstall it.
Lets you install addons marked as incompatible. This is great if you have a non-standard browser (like Pale Moon). I would like to see an option to disable the incompatibility warnings in the Add-on Manager and even allow you to automatically check and install updates in the Manager. Still, it works.
Until I get KeySnail *possibly* ported (rewritten with same functionality, but not using the code from original straight, just as reference) to the coming "new add-on api" - hopefully this api will support reading keypresses before firefox does anything to them, so can override any regular hotkeys... And there's always bound to be new add-on vs. firefox version issues, so this will be needed... currently many add-ons are incompatible with my Linux Mint 17.1 USB stick, which I boot from some systems...