I opened a new Mozilla bug tonight:

Bug 149794 – .exe appended to names of downloaded files

It’s probably a dupe, but I couldn’t find a bug like it in Bugzilla. I’ve seen the bug on all three of the computers I use since installing Mozilla 1.0.

Update: as I figured, the bug is a duplicate of bug 120327, which has about a billion comments. Even though the bug is old and well worn, I haven’t run into it, and I’ve been using Mozilla more or less exclusively for months.