$ ps -ef | grep thunder
jd       12600  3453  0 13:42 pts/4    00:00:00 grep --color=auto thunder

So, TB is not running at this point.

$ thunderbird --version

(process:3027): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Thunderbird 31.8.0

Using GUI:

Your Thunderbird profile cannot be loaded. It may be missing or inaccessible.

Yet, I cd $HOME/.thunderbird/

and I see the directory of my profile there.

I browse the directory, and all the files and dirs are there.

So, what has suddenly gone wrong?