our situation is the reverse of yours
we have the same difficulty but resizing does work for us.
the difference is that we are using Windows XP and FireFox 29.0 so they are much older then this application was developed for and we live with that.
the reason we do not use the "latest and greatest" as you are is for the reason you have seen not everything works. why on earth would you be using windows 10 on a machine that you make a living using for online sales. add to the the most current version of FireFox and it is no wonder there are issues. when you filled your support case did you not inform them about your operating system and FireFox version. the difference for you is that eventually they will support what you mention because others will have to move to it. Please take a bit of advice I learned in the 70-s don't be the first to use anything related to a computers let others be the "brave" ones and work out the problems. believe me when I say there is no on piece of software that dose not have a problem it is just that some are more server then others. If at all possible go back to windows 7 and an earlier version of FireFox