I have noticed dozens of times how tedious some websites are.

Therefore, I need a program, plugin for Firefox, or whatever that does the job - to be able to add some sort of checks to specific websites.

For example, the program could search for a regexp pattern, and then do an action according to it. For example if I find some language from the wikipedia page, I would like to move or copy it on top of that languages list. The action wouldn't necessarily have to be applied on the regexp it found: it could issue another regexp search, and if it's found, it would do the actions I want, such as delete a block of other piece of HTML, move it, or copy it to another location.