My only Operating System is Kubuntu 14.04 64bit LTS. The font changer will not install for my computer. I am old and need larger print to be able to read without strain. I am able to increase font size in the main screen, but unable to read the fonts in the menus and other areas where instructions are offered. Has anyone been able to solve this problem? Thanks for any help. 
Go to your profile folder>chrome folder (if you don’i find it create)>in chrome folder Chrome.css>/*all firefoxi fonts */
* {font-size: 15px !important;
}
(My answer applies also to Window & Mac not just Linux.)
Readers of this thread may also be interested in
- The helpful website Access Firefox which does list useful add-ons
Accessibility Extensions - The official Firefox help article Accessibility features in Firefox - Make Firefox and web content work for all users
I am aware of methods of changing the Firefox Menus etc using a preference { layout.css.devPixelsPerPx ] , but it is much easier to use add-ons .