My first post here - hoping someone can point me in the right direction!
I have been using Mint and Thunderbird successfully for a number of years now and love Mint. Previously, I've had no problems that I couldn't resolve myself but an issue has arisen which has got me stumped.
In the past, I have regularly attached files to outgoing emails from a usb stick. Following some recent problems with the computer (long story!), I was forced to do a fresh install of Mint and am now running 17.3 KDE 64bit and TB 52.4.0. I now find that, when I select the "Attach" button, the dialogue comes up but I am unable to browse to any files on the computer or usb. The dialogue looks like this:

the locations do not respond to clicking. All I can do is cancel and drag and drop files from Dolphin.
At first, I thought this was a Thunderbird problem but, when I came to upload a file to Dropbox, I found the same problem. Also, I have the same version number of TB running on my windows partition and that will browse and select files to attach fine.
I have looked through all the system settings and TB settings that I can find, and also tried to research on google but only found posts from years ago.
Any of you kind people have any idea what is causing this issue?