This has been asked at Stack Overflow and a similar question was asked at Super User. Here is a summary of the best answers:
Chandu Venjaramood: If the Web Dev Toolbar is installed, people can use this keyboard shortcuts: Command + Shift + S (Mac) and Control + Shift + S (Win)
yakunins: Install Adblock Plus, then add *.css rule in Filters options (custom filters tab). The method affect only on external stylesheets. It doesn't turn off inline styles.
Isxek: This Firefox Accessibility Extension allows you to turn off CSS when needed, and it also allows users to utilize their own stylesheet if needed.
ZTD: ...use Stylish (for Firefox or Chrome). Then get Stylish-Custom. ... You can set rules where any particular CSS file will turn on/off depending on what site you are currently viewing.
barfoon: ...[use] the personalized web extension for Chrome to accomplish this. It can block images/CSS for an entire domain every time you visit it.