After noticing the ubuntulooks_draw_progressbar_trough() call in Søren's stack trace, I tried switching to another theme.
While the "Human" control theme causes Firefox to crash on startup, Firefox runs without problems with the Clearlooks and ClearlooksClassic control themes (which ubuntulooks is based on).
So the problem most likely exists in one of the differences between clearlooks and ubuntulooks.
After noticing the ubuntulooks_ draw_progressba r_trough( ) call in Søren's stack trace, I tried switching to another theme.
While the "Human" control theme causes Firefox to crash on startup, Firefox runs without problems with the Clearlooks and ClearlooksClassic control themes (which ubuntulooks is based on).
So the problem most likely exists in one of the differences between clearlooks and ubuntulooks.