Slack Channels Grouping

Hyphens (-) and underscores (_) supported.
• prefix-name
• prefix_name

Slack Channels Grouping does not send the user's data anywhere. It's also completely open source.
https://github.com/yamadashy/slack-channels-grouping

For Firefox
https://addons.mozilla.org/en-US/firefox/addon/slack-channels-grouping/

After install, please reload your Slack page.