Upgraded to Firefox 23 August 6, 2013 on a Windows XP desktop when prompted. Aeon Jumbo theme I long enjoyed was not compatible and I chose an alternative large theme that came up on a search of "large". After a period of use I found the theme to be unsatisfactory and downloaded different large themes. However, they did not displace the first theme. Moreover, the first theme no longer shows in the list of add-ons but is still running. I cannot even switch to the default theme. I cannot tell you this theme's name because it will not come up in a search under "large" any longer either. Its description, to the extent I recall it, referenced visual accessibility intentions. The efforts to afford visual accessibility are to be lauded, but this is the second time the implementation has been half-baked and difficult to control. Previously, an earlier upgrade made web pages load oversize for my display. I use my monitor rotated into a portrait configuration, and if this is causing issues it needs to be accounted for in development. Thank you for your consideration. P.S. I implemented your troubleshooting add-on. The plug-ins listed may be accurate but they are not complete. In particular, I run DoNotTrackMe and NoScript which often get in the way of features.