Using the most recent Thunderbird, ditto Firefox, the system freezes frequently when I have T/bird and F/fox running at the same time.
F'rinstance today, I left T/bird running while I went to the BBC website. There I filled in an online comment and then tried to close Firefox, which declined to play nicely, and none of the usual rollover dialogue boxes on Mint functions worked - connection speed, state of battery etc just sat there glumly ignoring the mouse pointer. File and Quit on the function bar closed Firefox, but nothing would shift Thunderbird.
Three-fingered salute deleted the last two e-mailsin my inbox...
Had to power-down to regain control, but the e-mails seem to have vanished entirely. (Not in trash, nor in any other folder.)
Anyone any bright ideas? (Apart from not running Firefox and Thunderbird at the same time...)