How to remove old masking tape from glass?

Mix dish soap and water to make a VERY slippery substance. Then a common $2 razor scraper, being careful to make sure you change razor blades often and it never gets dull. If it gets dull it could scratch the glass.

The purpose of the soapy water is to make sure the razor blade glides across the glass.

It's quick work, unless the masking tape is still tacky. If it is, lay fresh masking tape across it, rub down and peel.

By the way, the razor blade method also works on primer and paint. You didn't actually need to mask the glass at all. You could have just overpainted the glass wherever you wanted to, and then used this same technique to lift the paint. It's actually much easier than what you're doing now.