You probably don't want to do that, as many weapons have heat that builds up and which eventually requires we weapon to stop for a time to cool off. Its better to have different groups of weapons to alternate between (plasma, missiles, rails, etc) so that you can fire just the ones you need in the situation.
If you are tired of holding down the mouse button, get (or make) some auto-firing turrets. Though they traditionally lose DPS when they autofire, there is a mod to disable that "feature" of the game.