Seems to work fine here in Firefox 7. If you press Ctrl+Shift+J after it locks up are there any errors? (You might need to press Clear before it happens to make sure other site's errors aren't showing)
In group Conditions Event 16 turn "ScoreChecker = 500" to "ScoreChecker = 50" => Takes less ships before being able to test the boss.
When boss appears=> FireBug error Spaceship_behavior.js l.322 this.dx is not extensible
Quick solution: Group "Bosses" Event 147 -> Toggle Stop SpaceShip
Long solution: Report the error on the spaceship movement topic and see if Fimbul can work it out.
This is for your capx. My guess is that the errors you get online are related. (Error: this.s is not extensible)
For the others, it looks like chrome errors. Error: this.showContextMenu is not a function Source File: chrome://y2layers/content/overlay.js Line: 34 ??? This is not part of firefox.
And for the last error I can't check for sure, because I will not download 16 Mo of datas with my slow connexion for now.
Now for the gameplay itself, I think your scores are a bit too high, as the boss was really the first challenge I encountered. The rest of the time it is just pressing right arrow and mashing my LMB (which cause my forearm to be hurtfull atm tbh :) ).
Make smaller waves, or smarter ennemies. Also, as mentionned, the size of the game is a bit heavy imo.