I do not like reddit's new UI - I liked the old and compact one a lot more; see the image for comparison
I kept getting annoyed by Firefox' autocomplete leading me to the 'www.reddit.com' instead of 'old.reddit.com' and I decided to modify my browsing history.
A quick search online pointed me to the docs for "The Places" database; which shows that you can simply edit a table in an sqlite db to modify your history
$ sqlite3 ~/.mozilla/firefox/*.default/places.sqlite > update moz_places set url = replace(url, '//reddit', '//old.reddit') where url like '%//reddit%'; > update moz_places set url = replace(url, 'www.reddit', 'old.reddit') where url like '%www.reddit%'; > exit
ensures I don't get redirected to the new UI anymore.