Slack works fine via TenFourFox, if a little slowly. Just check the option to spoof the user agent as Mac OS X on Intel in the settings.
For a lightweight option if the admin has enabled it you can use good old fashioned IRC for text chat via your IRC client of choice. I'd recommend Colloquy.
For a lightweight option if the admin has enabled it you can use good old fashioned IRC for text chat via your IRC client of choice. I'd recommend Colloquy.