Hi Makarovnik !
You could place(move) your extracted Waterfox as well as the " .mozilla " (a "hidden Folder on root) to your Drive and symlink then the (hidden) ".mozilla" Folder back to root ......and the waterfox executable back the to your running system and save this Configuration.
Means:
Your Waterfox Browser and the (hidden) ".mozilla" Folder are now on your Flash Drive not in changes Folder.......but still "Writes" will happen to your Usb Stick when using Waterfox. So there is not much saved....in my opinion.
I for myself did want to avoid as much as possible "Writes" to my SSD ......Running in Ram is good for it.
"Copy2ram" did help to accomplish my Goal.
Since having enough Ram....(8 Gigs)....i used the "copy2ram" Option in the Kernel Boot Line .
So i am running the Session completely in Ram....( just for those who have plenty of Ram).
(kernel /FOSSADOG64/casper/vmlinuz from=/FOSSADOG64 copy2ram noauto intel_pstate=enable changes=EXIT:/FOSSADOG64/casper/ ramsize=100% )
I still have Waterfox(portable) in /opt ....but now is completely running in Ram (fast as Hell). No Writings happens to my SSD during my Sessions while for Example surfing the web ---making updates and save only on Demand..then it is written to Drive.
Another few Apps i often use (Vlc-Smtube-Smplayer) as Squashfs Modules--- are placed in casper Folder---- they are loaded during boot-up automatically.......thus completely running in Ram....thus almost no "Writes" to my SSD during Session in Ram.....except i do a "save" on Demand or at Shutdown.
Hope i did it not to over complicated.
But feel free to ask.
Edit :
When I choose to save changes on shutdown is the entire /casper/changes saved to my flash drive or only files that have changed since the last save (full vs incremental backup so to speak).
Yes .( incremental backup so to speak)..if i correctly understood.....please correct me someone if i am wrong.
But you also should take a look into "QuickremasterGui" .....quite a nifty Tool.
Regards!