I will help if I can. At this point, I do not have an answer, but I might be able to find out the answer if you can work with me a little.

I have a question or two, and a suggestion.

Besides the slow startup, what do you see that tells you Firefox is leaking RAM?

When Firefox has given me problems, the first step always is the same. That step is to start Firefox in with addons disabled or in safe mode. Almost always, the problem disappears when started with addons disabled. That does not solve the problem, because I need the addons, but if the problem disappears when addons are off, that proves the problem is with the addons. Then, by activating one addon at a time, I have been able to find which addon is the culprit.

So, if have not done so already, you can restart Firefox with addons disabled by tapping as shown in the pictures attached to this response. Your can try safemode, by holding down the option key while starting Firefox.

Here is some info from Firefox that might help explain what I am trying to say:

Safe Mode is a special Firefox mode that can be used to troubleshoot and fix problems. Safe Mode temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes) that might be causing problems. By comparing Firefox behavior in normal mode to its behavior in Safe Mode you may be able to pinpoint the cause of the problem.

The Refresh Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Consider using it before going through a lengthy troubleshooting process.

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

I don't have Firefox add-ons. I've already tried the safe mode and not helping, Still taking memory and slowly startup. The slowness problem is only in the startup of Firefox.