Very good one... but it's a pity thjat this extension just exists in v1 and nothing happened.
So i've submitted a fork without the annoying "F3 to Close" function.
https://addons.mozilla.org/en-US/firefox/addon/quicktabs-light/

I like F1/F2 which are intuitive. But F3 - No, no, no! F3 is the Firefox 'Repeat Find' shortcut key which I use all the time, so accidentally close the tab. I accept that this extension is still in development because you acknowledge customization isn't possible - even though Options does show customize input boxes.

29Jul18 Update: V1.1 custom key binding doesn't solve problem of unwanted F3-Close behaviour. There is no information provided about what values can be entered, but by trial and error I found that if you type any value other than F1/F2/F3, when you press 'save' it is overwritten to the default F1/F2/F3. Suggested improvements:
1) Provide in-app guidance on entering custom keys.
2) Permit ALL F-keys: F1-F12.
3) Permit BLANK to indicate command not required: most likely to be used for CLOSE.
Developer's time and effort is always appreciated - thanks. For now, the alternative "QuickTabs Light" is my preferred app: https://addons.mozilla.org/en-US/firefox/addon/quicktabs-light/