Firefox performance on Linux significantly lags behind Firefox performance on Windows. Currently, measuring and drawing text stand out. The trunk backend for text wants to use Pango on Linux for complex text shaping and font selection, but not layout. Layout needs to be done inside the browser, but giving Pango a string to measure or draw requires going through its high-level calls designed for text layout. Using this method versus the old Xft backend results in a massive slowdown.
One solution is to avoid using pango for measurement on western text.
Profiles before and after:
Optimizing pango helps more. (fontset patch)
Insert pretty canvas graph here!