Use a fixed font for Gmail mail bodies
Feature added: April 19, 2006
There are lots of emails that rely on tabs and spacing to be easily read. Plain text table look awkward if you use a proportional font to view them. Some text in emails is so cleverly aligned that you're missing something essential if what you see is distorted, and some emails even include ASCII art, which is morphed into something totally new, though hardly better, when not seen through a fixed font.
Some Gmail users are also experiencing problem with certain type of emails where fonts are so small as to be unreadable. If you're having this type of problem, then this CustomizeGoogle feature will solve your problem.
Variable font (without CustomizeGoogle)
This feature will override the specific stylesheet classes that Gmail uses. It will not affect html email, but will allow text-only email to be displayed with a fixed-width font.
Fixed font (with CustomizeGoogle)
To use this feature you need to check the appropriate CustomizeGoogle option. This feature is disabled by default.
Firefox -> Tools -> CustomizeGoogle Options... -> Gmail