quicksave plugin has a compatibility issue with 1.5.2. It expects compose_bottom
hook in 1.5.2 to be same as in 1.4.x where output simply goes out to browser. While in fact compose.php was refactored to use templates and this hook was removed in https://sourceforge.net/p/squirrelmail/code/13540/.
Attached you can find a diff, which when applied to quicksave plugin, restores its functionality in 1.5.2 (note that you need to "Save" config in conf.pl in order for SquirrelMail to notice new hooks), while trying to maintain it for 1.4.x
Or, you can download updated version of this plugin from http://alexey.shpakovsky.ru/unlisted/quicksave.tar.gz