Adjust Firefox and Thunderbird to a High DPI touchscreen display (retina)

1. Install my Firefox-addon Zoom Menu Elements (or install this file from source on github).
   Alternative: you can manually open a new tab about:config in Firefox and search for layout.css.devPixelsPerPx set this value to 2 or 1.5 as you wish to adjust the size of the Firefox icons (source, bug report)

2. Install the Firefox-addon Fix for zoomed default-font to fix some issues with huge form elements on some pages (for ex. the login on reddit.com, ... see this question )

3. Pinch-to-zoom works out of the box (see here), for older versions the grab and drag addon for Firefox makes it possible to navigate the Internet by touch (disable it with the button in the add-on-bar when using a mouse)

4. Additionally, (but not needed) install the Firefox addon NoSquint or Default Fullzoomlevel and chose in settings: default full page zoom level 120%.

In Thunderbird you have to go for the alternative from step 1 in the advanced configuration (although I prefer not to change the overall size but use Ctrl+ and Ctrl- instead, to adjust the size of the email window)

I looks like Firefox 36.1 atleast, now picks up the scaling setting form Ubuntu. Its not dynamic - that is it reads the value when FF starts and its set at that until you restart FF. :)

AutoHiDPI is another good solution.