it works but slows my browser down

It breaks where a word you want to replace is part of a longer word. For example, replacing "car" wiþ "cat" gets you words like "catd" and "Catl" instead of "card" and "Carl". I tried fixing it by adding a space to eiðer end of every term I want to replace, but spaces are automatically replaced. Also, trying to replace a word wiþ two words, one of which is ðe original word, causes it to continue replacing ðe original word in ðe new phrase until you disable ðe extension. I tried to replace "tension" wiþ "sexual tension" and got ðis when a website tried to talk about a browser extension. https://imgur.com/n0jIVv0

Works as expected, simple UI, no complains.

With the latest firefox update, the addon slowly slows down the browser, otherwise it is a great addon, I hope the developer fixes it soon.

Excellent.
And Working fine!