That PPA seems to no longer exist, see here.
That other answer you linked to basically suggested installing the newest stable version of Thunderbird, which may have been more up-to-date than the latest stable version in the official repos at that time. But apparently, now the thunderbird package is updated often enough in the official repos.
Along the lines of the idea suggested in that other thread, if you have a similar problem and want try upgrading Thunderbird to solve it, you may try the Thunderbird Beta PPA:
sudo add-apt-repository ppa:mozillateam/thunderbird-next
sudo apt-get update
sudo apt-get install thunderbird
Be aware that this will install a beta version of thunderbird, with a higher potential for bugs.