Cannot attach Network Shares in Thunderbird

Suddenly (and I'm not sure if this was after a recent Ubuntu update or a Thunderbird one, Thunderbird no longer shows "Network" locations in the 'Attach File(s)' window. Normally below "Bookmarks" is another section "Network" which shows my network locations, however this entire section is now missing.

There is now no way of attaching a file existing anywhere on the network to an outgoing email. Nautilus can see and access all the network locations 100% fine. Drag/drop into the outgoing email from Nautilus will attach a network file to the email but Thunderbird refuses to send it and throws up a "Send Message Error":

Sending of the message failed.
There was an error attaching smb://mypc/my documents/myfolder/file.xls. Please check that you have access to the file.

Attaching network files in Thunderbird and on Ubuntu 14.04 has worked fine for two years. I'm a "user" and really not that au-fait with the inner workings of Linux so I hope this helps.

Thanks

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: thunderbird 1:52.1.1+build1-0ubuntu0.14.04.1
ProcVersionSignature: Ubuntu 3.13.0-119.166-generic 3.13.11-ckt39
Uname: Linux 3.13.0-119-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.14.1-0ubuntu3.24
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: jon 2048 F.... pulseaudio
BuildID: 20170512195315
Channel: Unavailable
CurrentDesktop: Unity
Date: Sun May 28 15:09:59 2017
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
InstallationDate: Installed on 2014-12-10 (900 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
IpRoute:
 default via 192.168.1.1 dev wlan0 proto static
 192.168.1.0/24 dev wlan0 proto kernel scope link src 192.168.1.7 metric 9
Locales: extensions.sqlite corrupt or missing
Plugins: Shockwave Flash - /usr/lib/flashplugin-installer/libflashplayer.so
PrefSources: prefs.js
ProcEnviron:
 LANGUAGE=en_GB:en
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=52.1.1/20170512195315 (In use)
RunningIncompatibleAddons: False
SourcePackage: thunderbird
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
WifiSyslog:

dmi.bios.date: 08/08/2013
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: X551CA.202
dmi.board.asset.tag: ATN12345678901234567
dmi.board.name: X551CA
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: 1.0
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: ASUSTeK COMPUTER INC.
dmi.chassis.version: 1.0
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrX551CA.202:bd08/08/2013:svnASUSTeKCOMPUTERINC.:pnX551CA:pvr1.0:rvnASUSTeKCOMPUTERINC.:rnX551CA:rvr1.0:cvnASUSTeKCOMPUTERINC.:ct10:cvr1.0:
dmi.product.name: X551CA
dmi.product.version: 1.0
dmi.sys.vendor: ASUSTeK COMPUTER INC.