CensorCAT от obviouslyweb

A dynamic and lightweight censoring extension for Firefox. Censor matching phrases on web pages using rules you define, and omit websites that you want to see unfiltered. Export and share your censor and omit lists with others. Curate the web how you want to see it.

CensorCAT is open-source; you can review the project's source code, submit issues and suggestions, as well as contact the creator here.