Are you tired of finding lots of google results of webpages that when entered require you to register to view the solution?
Most of that sites do dirty tricks: They check the requesting User Agent, to allow search robots/crawlers to see the full contents (and index them), but limiting the rest of users to only the question and not the answers.
Well, we can play dirty too and take advantage of Firefox's ease of customization.
We type "about:config", right-click, new "String" value, name "general.useragent.override", value "Googlebot/2.1 (+http://www.googlebot.com/bot.html)" (Googlebot's user agent string).
Save, try your new user agent typing "about:" (should be the new identifier), and then there you go, hidden parts now available.
You can test with this page for example (before and after tweaking the user agent).
Update: Ezequiel said at the comments that there is an extension to easily handle user agent switching, User Agent Switcher. Thanks for the tip!
Note: Be advised that changing the user agent to a non-common one may cause pages to not render correctly (or for example with limited JS or capabilities).