Using Firefox's Customize UI function works while FF is still running, but once you close it, when you reopen it there will be errors. Details of the problem and solution can be found in a review of the old Memory Fox at this link: https://addons.mozilla.org/en-US/firefox/user/roy192/
If it is removed by using the customize option, it will appear again next time I start firefox again, with two messages saying:
ReferenceError: BrowserToolboxCustomizeDone is not defined
and
- [ Memory Fox ] failed to add button to toolbar.ReferenceError: BrowserToolboxCustomizeDone is not defined
There is a hack mentioned below this that I tried & works, but just my luck, FF48 released & it completely disables overriding unsigned extensions, so the Memory Fox Next is sitting there disabled.
a solution to this problem by going to \chrome\content\mfxOverlay.js and replacing the 559th line:
if( !_branch2.getBoolPref("tlbBtn") )
by
if(0)
If I may suggest, Memory Fox really doesn't need a button. If you can move the options available in the button's menu & simply remove the button altogether it would be great. That is, if you're still working on it at all. Either way, thank you for having provided an update of this essential extension.