That is a built-in Firefox container feature. Open a website in a container and then click on the container icon (right next to the search bar) to keep that site open in that container. Every other time, that site loads directly into that container
What's great about regex matching is that you can grab a regex for each of your Google accounts, and bookmark the URL that the regex matches. I've changed the Firefox search bar config to prioritise bookmarks in the results. I can now type 'gmail', and the top two results are:
Gmail (work)
Gmail (personal)
Each opens in opens in the correct container. The trick sometimes is finding a bookmarkable URL with a specific string that you can use in a regex.
To be honest it's one of the features I wish Firefox would add to their add-on because I don't like giving their party add-ons access to my data.