Firefox Hangs Laptop Followed by Unbootable System

[Nick Evans]

I've got a Dell XPS13 2015 running 5.4.1 with the latest packages from
a few days ago. I compiled Firefox last night which managed to hang
the system within a few minutes (if I can fix this I'll move on to
that problem next). The system has been otherwise stable in X, but it
completely locked up with Firefox. I couldn't get back to the terminal
so I powered off the laptop for the night and this morning was greeted
with:

da0: supports TRIM
Mounting root from hammer:serno/S1D3NYAG209909.s1d
HAMMER(ROOT) recovery check seqno:00e1fa5d
HAMMER(ROOT) recovery range <guid>
HAMMER(ROOT) recovery nexto <guid>
HAMMER(ROOT) recovery undo <guid> (536554448 bytes)(RW)
HAMMER(ROOT) Found REDO_SYNC <partial-guid>
HAMMER(ROOT) Ignoring extra REDO_SYNC records in UNDO/REDO FIFO.
HAMMER(ROOT) recovery complete
HAMMER(ROOT) recovery redo <guid> (536554448 bytes)(RW)
HAMMER(ROOT) Embedded extended redo <partial-guid>, (-536554448 bytes)(RW)
HAMMER: FIFO record bad head signature 0000 at <partial-guid>
HAMMER(ROOT) Illegal UNDO TAIL signature at <partial-guid>
HAMMER(ROOT) End redorecovery
Root mount failed: 5

followed by the dreaded mountroot> prompt. Excuse the <guid> nonsense, I
have no good way of getting the log over to another PC except as an
image so I manually transcribed it. The only references I've seen to
similar errors are from 2012/2013 and I didn't see a remediation other
than kernel patches. This is a default partition setup by the installer
on a Samsung PM851 250GB SATA SSD. So far this was merely a system I've
been tinkering with. If I have to nuke it that's fine, I can mount the
drive read only via the installer media so I can at least copy off my
configs if necessary.

Thanks for any pointers,
Nick