There are two solutions:
1.
Click on the little white Strokeit arrow in the system tray to open the 'Strokeit Command Editor'.
Then click on 'Firefox' or 'Mozilla' on the list at the left.
Then click 'Disable gestures in this application' at lower right.
2.
Alternatively you could create all sorts of gr8 gestures for Firefox (along with the 'global' ones), which I've found to be even better and more versatile than the 'All-in-one' extension.
As an example you could look at some of mine in the attached .sxp file