I've switched from Firefox to Edge in the past year and it's been a marked improvement. I like the cross platform compatibility and syncing works great among Mac/Windows/iOS. Edge started really poorly but has developed into a first class browser, and combining the privacy enhancements of Firefox with the performance of Chrome.

At this point, not sure why I would switch back to Firefox TBH, unless MS really screws something up. My advice would be to create a seamless cross platform browsing experience that has feature parity across all devices. Keep the privacy first strategy, and look at incorporating key add-ons (could they purchase a Bitward/1Password/LastPass?).