Thanks again for a reply.
I was first of all trying to find the best place to discuss this. I can use the various Firefox forums on Mozilla, and in fact this problem as I am seeing it at present is rather specific to those forums.
I will refer back to that site. It appears it is a site problem. Epiphany and canonical Firefox render the page in a similar way.
My initial thoughts were:
Most users on those forums do not use Linux, so I thought I may hit a bigger Ubuntu audience by asking here, but expected to be told of a better thread or section to post in.
I have attached a thumbnail screenshot. The screen has two browsers open Lefthand side is Aurora canonical. Righthand side is Mozilla's own version using a recent Nightly.
You will notice Nightly displays messages as intended. It displays the users icon, then has the remainder of the screen width to display the message, it shows only the first part of each message, until an individual message is opened. Although if the message is short, it may all fit in the preview.
The lefthand browser is indenting all the messages. It is wasting half of the screen space because of this.
I have tried the usual troubleshooting. Initially I thought it likely to be a regression in the code changes on the message site, although that seemed unlikely as no one else was mentioning it. I then noticed I did not have the problem booting in Windows. Now I see I only have the problem using canonical editions.
I have gone further and also noted that I may save the page source and view it offline. When viewed offline in Mozilla own versions it displays as expected. When viewed in canonical it wastes space.
I do not know how to run multiple copies of canonical, but problem has been seen in at least Firefox (11 ?) 12 and currently as per the screenshot in 15 [Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:15.0) Gecko/20120615 Firefox/15.0a2 ]
I do not understand HTML & CSS but guess either the canonical version is doing something odd, or the site code is doing something odd when it picks up the user agent string. The site is https://support.mozilla.org/en-US/messages You will need to register and have messages to see the problem. (Official Firefox support site, free for anyone to register, my user name on that site John99 )