Dangerous! Somewhat... Rated 4 out of 5 stars

The plugin has a nice interface and great options, if it only did the job. I need to replace a repetitive word in some 250+ Youtube video descriptions. I entered the rule and checked it to be applied on page load.
The I clicked to edit the description, the plugin replaced the text.
When I checked the video page, the text was also replaced. However, as soon as I disabled the plugin - the text was back to the original.
That means, at least for youtube, the plugin just changes the display of text in your own browser, but doesn't fill the form.
And that is very dangerous, because you may be sure that you've changed everything while in fact, you didn't.
The potential is great, but while the plugin just censors some text for your eyes only, it's useless.
And one more thing. On plugin removal the extensions.foxreplace.substitutionListJSON is still in the config, so the rules are all back if you reinstall the plugin. I don't know if it's a bug or a feature,

PS: MarcRA, thank you for replying so fast. To enable Save in Youtube you must fiddle with the text. After the replace is done, I made some other alterations and click save. The plugin-replaced text doesn't save in the form. The danger lies in that your browser shows an illusion, and other people see the real thing.
However, I found a work-around by switching to the mobile version of the site. And this time, the plugin works! So I guess it's the way how desktop Youtube site treats text input that's yet to be implemented in the plugin. Anyway, the workaround should still save a lot of effort, so thank you so much for the plugin.

This review is for a previous version of the add-on ( 

Did you click the "save" button after editing the form? If you didn't, then of course it didn't do any permanent change. If you did, then it may be a bug and you should report it at https://code.google.com/p/foxreplace/issues/list .

On the other hand, I don't see how this can be "dangerous".

PS: OK, I'm happy you could find a workaround. I'll take note on this problem to see if it can be solved.