On your local terminal please type the below commands

cd ~/.mozilla/firefox

ls -l
drwx------ 15 cosd cos  4096 Jan 28 15:49 6riqoa5d.Room 80-1526463020896
drwx------ 18 cosd cos 12288 Dec 13 14:22 73gfnoyw.New
drwx------  5 cosd cos  4096 Jan 18 14:00 Crash Reports
drwx------  2 cosd cos  4096 Mar 26  2018 Pending Pings
drwx------ 11 cosd cos  4096 Dec 13 14:22 bttl2p8i.Default User
-rw-r-----  1 cosd cos    27 Apr 27  2016 profiles
-rw-r-----  1 cosd cos   249 Dec 13 14:22 profiles.ini

This will display your Firefox profiles, you want to open the most recently used profile, so in this case folder: "6riqoa5d.Room 80-1526463020896"

cd 6riqoa5d.Room\ 80-1526463020896/

(\ is required if there is a space in the folder name)

There should be one or more lock files you will need to remove:

ls -la | grep lock
-rw-r-----  1 cosd cos        0 Jan 28 09:28 .parentlock
-rw-------  1 cosd cos   192549 Jan 24 09:23 blocklist.xml
drwx------  2 cosd cos     4096 Jan 24 09:23 blocklists
lrwxrwxrwx  1 cosd cos       21 Jan 28 09:28 lock -> 136.156.192.57:+32704

# remove both .parentlock and lock files if they exist

rm lock .parentlock

You should now be able to open firefox, if there are any problems please contact servicedesk