Hi David,
While not exactly a matching combination to yours, I do have Thunderbird 45.8.0 and Firefox 45.9.0 running side-by-side all day long, day after day.
It is usually Firefox that is the first one to exhaust all available memory and therefore needing a re-start. Normally this happens once I see over 1Gig of memory use in Theseus. A tell-tale sign is that page rendeing in Firefox ends up slowing down until it is just faster to re-start and be back up to speed.
So here is what has brought a good amount of stability to my system: AOS 202 SMP kernel. I'm not saying this is the "super fix", but I will describe the BEFORE & AFTER situation.
Just for the record I am running a Warp4 CP2 SMP box here, this is not a AOS install, rather a progressively updated installation. As you can imagine there are some warts and uglies with this approach, so certainly not as slick as a clean AOS install. I am also using the YUM/RPM system.
Further on, both FF and TB DLLs are marked to load-high using 'C'ode only option.
BEFORE
=====
1) I used to pre-load the FF and TB DLLs during system boot
2) This prevented the excessive memory consumption when FF or TB would be re-started without a complete system re-boot
- following about 2-3 days of such re-starts enough memory would be leaked/fragmented that re-starting FF would no longer recover to a usable state
AFTER
====
1) deployed the AOS 202 SMP kernel (update from 201), one of the key fixes there was the quashing of the bug which prevented previously loaded DLLs from be successfully un-loaded (and therefore memory released) when not in use
2) I no longer pre-load the FF and TB DLLs during system boot
2) The applications run reliably for about 5-7 days before the shared memory is so fragmented that no other apps can function because a sufficiently large consecutive chunk of memory can no longer be allocated
- this situation requires a re-boot
What is your configuration?