To those who comment(ed) and/or complain(ed) about this bug:

1- How many of you have smooth scrolling on?

2- "dead slow", "horrible", "unbearable", etc.. are very subjective adjectives which do not give any kind of accurate measurement, objective measurement, quantifiable data, comparable data like a performance profile comparison. 0.3sec could be deadly slow to you while 0.4sec could be just fast enough to person_B. We have no idea here. How big is the gap between horribly slow and fast enough in milliseconds? We have no clue, no idea, not even an hint. And we have to compare (and to assess) all these measurements regarding respective hardware, configuration, os, settings, etc... involved.

3- How long is a long table or a very long table? "very long" is still vague, not quantifiable, non comparable, non-specific, not really useful ... and I'm not even mentioning nested tables, deeply nested tables and over-excessively formated tables for layout purpose here..
"scrolling performance regression for very large tables but it has nothing to do with THIS bug". Dimitrios, comment #110

4- How many of you are mentioning webpages which have hundreds of validation markup errors, CSS errors, regarding webpages which can not in any way constitute a reduced testcase highlighting the issue in this bug?

5- How many of you are mentioning webpages that have lots of images, which are Flash-intensive, Flash-dependent, with a very large and deep DOM tree of nodes, with divitis and classitis, with thousands of javascript lines of code spread into several script files and functions, abusing setTimeout and setInterval, with generated content (like ads rotating, iframe refreshed), in over-constrained layout, over-excessively defined constraining stylesheets for pixel-perfect layout, etc.?

6- How many of you are mentioning webpages which would be considered - anyway and regardless - CPU-demanding, RAM-demanding, video-memory-demanding and user-system-resources demanding according to today's standards? How many of those webpages are already pushing the limits a bit far to begin with?

7- "'position: fixed;' is very importan for top menu on intranets": Do intranet webpage require 100 fixed elements?

8- How many of you are referring to characteristics which should be reported in other bugs (bug 90198, bug 64401, bug 202718, bug 54542, etc...) and not in this bug?

9- How many of you have read
What is a Simplified Test Case, and How Do I Make One?
http://www.mozilla.org/newlayout/bugathon.html#testcase

10- How many of you have read 
How to Really, Really Help Developers on Bugs -- Minimal Testcases
http://wiki.mozilla.org/MozillaQualityAssurance:Triage#How_to_Really.2C_Really_Help_Developers_on_Bugs_--_Minimal_Testcases

11- How many background images (tiled) constitute the fixed background regarding the mentioned webpages? You do understand that one big image in the background is quite different from having 1px by 1px image repeated+tiled all over the canvas... Anyway, why such issue is mentioned in this bug and not instead bug 90198 .. if that is needed?

12- How many of you have actually read the Bug writing guidelines?
http://developer.mozilla.org/en/docs/Bug_writing_guidelines

13- How many of you have actually read the bugzilla etiquette?
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html

14- http://firefox.bric.de/index.php mentioned in comment #39 is 403 Forbidden; <a href="http://www.n3t.nl/mozbug/">minimal tescase</a> mentioned in comment #74 is redirected; etc.

15- http://www.theloudspeakerkit.com/shop/ has been mentioned in comment #112: a closer examination of that page reveals flash-dependency, lots of mouseover script dependency, general bloated markup code, 4 imported stylesheets (with one declared twice), over 700 lines of CSS code and not even one, not one, position: fixed element.

16- http://abiads08.ab.funpic.de/test123/index.html mentioned in comment #118 is not at all a recommendable webpage (general bloated code, lots of errors, ads-embedded) for this bug. In fact, after carefully checking, there is no position: fixed elements found anywhere. background-attachment: fixed may be about bug 90198 like others have said.

Once a bug has been confirmed and is clearly defined, well understood, there is very little one can say that will actually help besides 
a) submitting a patch or 
b) funding/subsiding a fix by developers.

When I try attachment 139911 with today's trunk build, I can not notice any slow scrolling issues worth mentioning. Same thing with the provided URL. Same thing with the so-called "dead slow", "unbearable" webpage
http://www.vanwensveen.nl/rants/microsoft/IhateMS_2.html mentioned in comment #78. And I have a modest P3 667Mhz, 384MB RAM on Windows XP SP3. And this bug is filed for trunk, not to the 1.8 or 1.9.0 branch. And even in 1.8 and 1.9.0 branches, I notice nothing dramatic or obviously slow.

To me, this bug should be resolved as WFM and shouldn't be reopened without excellent reasons to and with many conditions met.

Regards, Gérard