Found it!
Split out the intro code and got confused 'cos it worked on it's own.
Turns out offending piece was unrelated: the filter property in the body element seems to not play well for unknown reason (probably a bug for the Firefox team ;)). Removed it and all seems well.
Found it! Split out the intro code and got confused 'cos it worked on it's own. Turns out offending piece was unrelated: the
filter
property in the body element seems to not play well for unknown reason (probably a bug for the Firefox team ;)). Removed it and all seems well.