From which version of the extension did you upgrade to v5.2? Are there any error messages being displayed on the console? Which platform are you using? Thanks.
Addendum: OK. I still cannot reproduce the problem. In about:config, are there any keys starting with "extensions.easygestures"?
Addendum 2: I have upgraded from v5.1 to v5.2 on a Win10 machine with Firefox 54, and I experienced no problem. Does your problem persist after uninstalling and reinstalling the extension?
The presence of preference keys starting with "extensions.easygestures" is a clear sign that something went wrong during the upgrade procedure (as from v5.2 there shouldn't be any such keys in about:config anymore). I would try to uninstall the extension and manually delete all these keys starting with "extensions.easygestures" (alternatively, you can downgrade to v5.1 and then uninstall that version of the extension). At this point, if you search "easygestures" in about:config no item should be returned. Is that so? If yes, I would expect that installing v5.2 solves the problem. If not, which keys contain still that term?
Do I understand correctly that no horizontal slit window is shown anymore and that a blank tab is shown instead when opening options? That represents some progress...
I am currently using the 64-bit versions of Firefox 55 and 56, and the extension runs without problems.
BTW, which version of Firefox are you actually using. Version 5.2 of easyGestures is incompatible with Firefox 50.1!
Great to read that you got it to work! Indeed, the pie menu cannot be opened on addons.mozilla.org. This is a limitation of WebExtensions.