Is your concern just for reading incoming messages? Or are you also wanting to change font for outgoing messages?

globally everything default view when I look at all my messages in inbox that page

ok, click settings>general and select the preferred font and font size. For more detail, click the advanced button. You will see additional options there, such as allowing other fonts or not, and which font for plain text messages, if a different one is desired.

The font for the folder pane, message list, menus etc. can be set with css:

/* global serif font */
* { 
	font-size: 10pt !important;
  	font-family: Cambria, serif !important;}

/* global sans-serif font */
* { 
        font-size: 12pt !important;
  	font-family: Open Sans, sans-serif !important; }

Use the code for either serif or sans-serif.

Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code, change the number and font as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

video on how to create a css file (Firefox and TB)