I use thunderbird from thunderbird-next ppa (currently 15.0~b4+build1-0ubuntu0.12.04.1~mtn1) and I observe the same behaviour. When deleting messages (my daily dose, about a hundred), ie. moving them to trash folder, the memory consumption is 1.7GB on 4GB machine. I use IMAPs for my mail accounts.
I use thunderbird from thunderbird-next ppa (currently 15.0~b4+ build1- 0ubuntu0. 12.04.1~ mtn1) and I observe the same behaviour. When deleting messages (my daily dose, about a hundred), ie. moving them to trash folder, the memory consumption is 1.7GB on 4GB machine. I use IMAPs for my mail accounts.