You can do that with https://addons.mozilla.org/en-US/firefox/addon/redirector/

There's an extension I used to use called 'StayFocusd'. It's intended for blocking distracting websites, you choose your only rules, and it's really strict about enforcing them - you can only change blocking rules on days that aren't today, for example, to stop you from temporarily unblocking a website. You can either block them completely, or give them a daily limit.

You could install that and put Reddit on it. If you really need Reddit for something (say, getting a relevant search result in a sea of SEO), you'll still be able to bypass the extension to view it. But it will make you type a paragraph accurately to make sure it's a conscious decision. You can't simply disable the extension.

Use redirector! Here's one i will copy paste for everyone to use:

Description: reddit bye bye
Example URL:https://www.reddit.com/*
Include pattern:https://www.reddit.com/*
Redirect to:https://lemmy.ml/$

Make sure to replace lemmy.ml with your instance of choice and reddit.com with old reddit if you default to that (I always had "use redesign" turned off in my account settings).

For bonus fun, you can copy redirects with the extension, so you can adjust for your favorite subreddits if you choose.

https://addons.mozilla.org/en-US/firefox/addon/redirector/

boom