Don't execute focus tests in Firefox

Close gh-1423
Check Gruntfile and tasks for code style
Remove workarounds for the uglify task mishandling banners when used with source maps.

The issue was fixed in grunt-contrib-uglify:
https://github.com/gruntjs/grunt-contrib-uglify/issues/22
No ticket. Restore some parsing tests in core.
Fix #14415. Remove the source map comment in the release script. Close gh-1424.
No ticket. Remove the pre-uglify task form build/tasks/build.js. Fix custom builds.
No ticket. Remove version sniffing from test/unit/support.js, browsers got updated.