How do I search a page using a regular expression in Firefox?

Since the other alternatives offered here have been left in the dust of technological change, and rendered incompatible with latest versions, I'll drop the two current compatible addons I found (though I haven't tested them):

• Regex Search
  Search in HTML or Highlight using Regular Expression (REGEX) and output the results using template, with saving profiles functionality, you need to know 'basic' Regular expression to benefit from this plugin
• {find+}
  {find+} is a simple Find-in-Page extension hosting a the ability to search a web page or document by regular expression. It has been designed to look and behave much like the CTRL-F function, but extended to provide a feature-rich and robust user experience with the aim of completely replacing the native Find-in-Page tool.

Fastest Search

Best text search on page/in tabs - count/regex/match distribution/list/find-as-you-type. Fastest Instant web/history search in URLBar. Best custom search and multi-tab preview in any box or for selected text. Autocopy, dnd link/image.

Yet another option matching my needs very well is Regex Find which is open sourced. It's simple, though it may not be so polished.