While I remember time, when Netscape Navigator was renamed to Mozilla, after Mozilla I've been using only Firefox 3, 19 and 45, only upgrading when it was unsustainable to use the old version... (And I'm not going to use any newer Firefox version, since it does not support saving MHT files, which I crucially need... Great thanks for the Palemoon browser, which is now more "Firefox" than the new Firefox itself... I keep using Firefox 45 only marginally for administrative tasks and few forums where it remembers the login I don't, doing all news reading in Palemoon...)... is it anything we introduced, or is it something Mozilla did before our fork point? You keep mentioning Firefox 45...
From what I can tell, the Firefox 45 has the same inefficient jemalloc as Palemoon, just it endures a little more, not any much more, probably because some objects were smaller...??
To further illustrate the problem, the attachments are from about:memory shortly before the crash, when Palemoon started to have repaint problems...
There were 554 Mb "reserved", i.e. decommited memory, wasting address space... (while it's right to decommit unused memory, so that it spares physical memory, but it must be reused, when new memory is needed, otherwise address space clogs, as there are only 2Gb available in 32-bit version...)
On the last line of second attachment, it shows there was only 13 Mb max "contiguous" space left, all rest of address space clogged...
Also the difference between heap-allocated and heap-mapped shows a lot of memory wasting, not reusing free blocks properly...
(this is from Palemoon version 32.2.1 but I doubt it would differ in newer release...)
πα½