This extension allows you to replace text fragments (strings) in a page by other text fragments. The system is based on substitutions, where each substitution has an input text which has to be replaced (the "Replace" field) and an output text by which the first has to be replaced (the "With" field). When a substitution is applied it's over the whole content of a page (you can't do partial substitutions at the moment). Substitutions can be case-sensitive or insensitive. The use of regular expressions is also supported.
You can do individual substitutions or define a list of substitutions to apply all at once. You can define substitution lists that apply only to pages that match an URL. You also have the option to apply the substitution list automatically whenever a page is loaded.
Possible future features: RegExp editor, undo, automatic substitutions on AJAX pages, etc. These are ideas, but it doesn't mean that all of them can be implemented.