Why is this happening in root? You should normally run Firefox as a user and have the profile in your home directory (~/.mozilla).

Did you check the size of each folder in .mozilla to see which of them takes up the most space? Do you only have one profile? Possible candidates are the storage/default folder or a cache2 folder although this folder would normally be in "~/.cache/mozilla/firefox". If you have a lot of bookmarks then also check the backups in the bookmarkbackups folder.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

What might be a reasonable size to expect? No, I only have one profile. How can I purge the thing? I'd expect all I need to keep are my bookmarks, settings, open tabs, menubars, cookies ... what else? There are a zillion files that look like this:

[ 4096] 1089929928m7e4s2s4e4n2g7e6r0_8w.files/ [ 139264] 1089929928m7e4s2s4e4n2g7e6r0_8w.sqlite [ 4096] 1244373001m7e4s2s4e4n2g7e6r0_8w.files/ [ 49152] 1244373001m7e4s2s4e4n2g7e6r0_8w.sqlite [ 4096] 1913780088AesgyanrcoSt.files/ [ 49152] 1913780088AesgyanrcoSt.sqlite [ 4096] 2498542862____ttes.files/ [ 49152] 2498542862____ttes.sqlite [ 4096] 3172861392m7e4s2s4e4n2g7e6r0_8w.files/ [ 49152] 3172861392m7e4s2s4e4n2g7e6r0_8w.sqlite [ 4096] 3282182777SeegravriocteSWconryk.files/ [ 49152] 3282182777SeegravriocteSWconryk.sqlite [ 4096] 3517821712sktcoerhacg_ea_tqou.files/ [ 4096] journals/ [ 49152] 3517821712sktcoerhacg_ea_tqou.sqlite [ 4096] 3670119413m7e4s2s4e4n2g7e6r0_8w.files/ [ 49152] 3670119413m7e4s2s4e4n2g7e6r0_8w.sqlite [ 4096] 3713173747_s_edmban.files/ [ 49152] 3713173747_s_edmban.sqlite [ 4096] 3723501555m7e4s2s4e4n2g7e6r0_8w.files/ [ 57344] 3723501555m7e4s2s4e4n2g7e6r0_8w.sqlite [ 4096] 708916446mbwd__nsontoiiftiac.files/ [ 49152] 708916446mbwd__nsontoiiftiac.sqlite

... and this:

[ 4096] morgue/ [ 4096] 10/ [ 1508] {516ea9bf-9ba2-4e9a-a60d-90b8253e250a}.final [ 4096] 100/ [ 6326] {3cd94597-8032-418b-ae13-934ff8cec264}.final [ 4096] 101/ [ 3534] {901d4ee3-ce7d-47f5-b01f-ec856d41f765}.final [ 5284] {91a9f001-f8ca-4fe4-b2bd-13c2e6ff7465}.final [ 4096] 102/ [ 2796] {49bb1380-a487-4f78-9d2e-d54879fff766}.final [ 4096] 103/ [ 17997] {2830bde4-1acf-4e70-b5bb-2733bce29f67}.final [ 4096] 104/ [ 2617] {79fab0e7-1320-41f8-a448-a887fe4a2268}.final [ 1810] {cac75bc5-445d-471a-bf2a-acb6e0db9868}.final [ 1636] {e010fa4e-1073-47ca-837f-c42f94b3e668}.final [ 4096] 106/

In this world of looooowwww price storage.... Are you on a desktop or laptop? I run all versions of Firefox and Waterfox from my Downloads folder and you can see the size. see screenshots I would suggest you download Firefox from Mozilla to different location, sync it to your current install from your distro, verify everything has synced, and then zip and move that folder from root to save as a backup. Also, buy more storage.  ;-))

It's not that I don't have the room, it's that I get the feeling that 99% of that stuff is superfluous. I'm a minimalist by nature. Anyway you are using roughly half the size, so I'm in the ball park. I wonder if syncing to a new install would trim out some of the bloat? If so, I'd go for it.

That would be a good test and it's good to have your data on different devices and versions in case something goes BOOM.

I'm the most backed up person on the planet :-)