I noticed the difference in vertical alignment on the angle-right and angle-left icons ('\f105' and '\f104') in Firefox. I haven't seen this problem in other browsers I've tested in. I can adjust the alignment in the stylesheet on a case by case basis, but it would much better if Fontello could be dropped in as a replacement for Font Awesome without needing fine-tuning like that.
Changing the "Units per em" and "Ascent" font metrics to match Font Awesome (1792 and 1536, respectively) didn't seem to make a difference.
Anyway, thanks for providing this great tool/resource.
I'd suggest 2 things:
If that does not help, then metrics compare with ttx utility required.
Strange, the only way I had nearly similar results across browsers was to set baseline to 0...
(Font with 5 social media icons from FA)