I know this is Lemmy and plenty of users here are anti-Reddit, but I'm sharing this in case other users have trouble with this as well.

Edit: u/Capta1nT0ad commented a working regex rule, Idk why my attempts didn't. Anyway I modified the rule so it works only on Reddit u/ and r/ links, so it doesn't have a chance of touching links that aren't compatible. I made a pastebin for anyone that wants to just import the file.

{
    "createdBy": "Redirector v3.5.3",
    "createdAt": "2024-05-02T08:36:19.092Z",
    "redirects": [
        {
            "description": "Redirect to New Reddit, or old Reddit by replacing new.reddit.com in the redirect URL to old.reddit.com",
            "exampleUrl": "https://www.reddit.com/r/reddit",
            "exampleResult": "https://new.reddit.com/r/reddit",
            "error": null,
            "includePattern": ".+?:\/\/www.reddit.com\/(((r|u|user)\/.*)|$)",
            "excludePattern": "",
            "patternDesc": "",
            "redirectUrl": "https://new.reddit.com$1",
            "patternType": "R",
            "processMatches": "noProcessing",
            "disabled": false,
            "grouped": false,
            "appliesTo": [
                "main_frame"
            ]
        }
    ]
}

A redirector addon is necessary to redirect Reddit pages from the new 2023-24 redesign to either the 2018 redesign or old Reddit*. Some pages can't be redirected, like https://www.reddit.com/prefs/feeds/ and https://www.reddit.com/mod/firefox/queue (the new mod queue, firefox being the subreddit name).

When I go those links, the following addons don't work for them: Reddit UI Redirector, Old Reddit Redirector, and Old New Reddit Redirector. New Reddit Redirect doesn't even work at all for me, though it hasn't been updated since 2019 and there doesn't seem to be a repo online, so its probably close sourced. I left reviews for all of them. The only addon I found working both links is UI Changer for Reddit.

* I tried making a regex rule with the Redirector addon but it didn't work.