lsblk is just lacking a lot of information and creating a false impression of what is happening. I did a bind mount to try it out.

sudo mount -o ro --bind /var/log /mnt

This mounts /var/log to /mnt without making any other changes. My root partition is still mounted at / and fully functional. However, all that lsblk shows under MOUNTPOINTS is /mnt. There is no indication that it's just /var/log that is mounted and not the entire root partition. There is also no mention at all of /. findmnt shows this correctly. Omitting all irrelevant info, I get:

TARGET                                                SOURCE                 [...]
/                                                     /dev/dm-0              [...]
[...]
└─/mnt                                                /dev/dm-0[/var/log]    [...]

Here you can see that the same device is used for both mountpoints and that it's just /var/log that is mounted at /mnt.

Snap is probably doing something similar. It is mounting a specific directory into the directory of the firefox snap. It is not using your entire root partition and it's not doing something that would break the / mountpoint. This by itself should cause no issues at all. You can see in the issue you linked as well that the fix to their boot issue was something completely irrelevant.