Recently all toolbar icons in Firefox have been converted to SVGs [1] and in the process several performance problems were found and fixed or are in the process of being fixed [2]. You may want to try out a recent Nightly build.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1347543 [2] https://bugzilla.mozilla.org/show_bug.cgi?id=1054016