Buggy or corrupted install of Waterfox perhaps:
Have you tried downloading, reinstalling, and reconfiguring Waterfox?
How many tabs do you have open?
Start with one tab open, then watch the Task Manager window as you continue to open other tabs.
Determine if there is some threshold number of tasks when the memory starts getting or gets consumed.
And keep an eye on what all is being launched at start up. Something there could be grabbing and holding/ not releasing memory.