Firefox has been solid for me since the dawn of time, and I have or currently run it on Windows, Mac, Linux, and FreeBSD.
For a while it used to be kind of slow on Mac and Linux, but I think that was slow graphics calls, which points to a possible issue with the graphics driver I was using. But the last time I checked (many months ago), it was much better.
For a while it used to be kind of slow on Mac and Linux, but I think that was slow graphics calls, which points to a possible issue with the graphics driver I was using. But the last time I checked (many months ago), it was much better.