User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9b5pre) Gecko/2008030802 SeaMonkey/2.0a1pre Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9b5pre) Gecko/2008030802 SeaMonkey/2.0a1pre I noticed very slow handling of some actions in the browser for quite some time and put some effort in finding the reason last month. By trial and error I found the extension prefbar to cause the slowdown (see https://www.mozdev.org/bugs/show_bug.cgi?id=18655 for discussion). Lowering down the problem it doesn't seem prefbar specific but caused by just adding an overlay. While testing this on current builds I then noticed that the slowdown on the page I use to test the effect (sorry, no simple testcase) also shows without any plugins, even in a fresh profile. I then narrowed down the time window to firefox trunk until 20080215 being fast and > 20080216 being slow. The time window for seamonkey trunk is one day behind: <= 20080216 being fast and > 20080217 being slow. Reproducible: Always Steps to Reproduce: 1. Load the above URL 2. Move the mouse over the links of the threading. Actual Results: I've to move the mouse quite slow so no line fails to show the hover view. If moving faster the hover will lag behind and skips links. Expected Results: Also on fastest moving the hover should be shown for each and every line without lag. And that's the way it works when using a older build or moving not over the links but elsewhere over the lines (in the name or date column). I guess the visibility depends on the hardware. Mine is about 7 years old, but hardware isn't the issue since it all works ok with older builds.