Autoþorn автор Firefly360

Enable replacement - starts replacing th on your visited webpages with your choices of þ and ð.
Use doubling - doubles þ or ð when between two vowels.
Also use ð - uses ð rather than þ for the voiced version of th (as in THat).
Harder dialect - uses the voiced version of words for replacement when different by dialect - e.g. wiþ --> wið.
Custom glyph output - specify unique characters (e.g. runic symbols) to use in place of standard ÞþÐð.
---Added v1.1 ---
{Firefox Support}
Add new entry - Use to specify replacement rules for a th word that isn't being replaced
Edit entry - Change a preexisting replacement or manually entered replacement to new rules