I created a new group with a blank tab switched to it and when tried to switch back to the original group with all the tabs they were all gone. I'm using firefox 58 beta x64 on windows 7 x64. It would be nice if you open a github page to be able to leave you feedback/bug reports there.
I'm very sorry that this happened. Could you update to version 1.5 and check again this bug? Github page: https://github.com/Drive4ik/simple-tab-groups
