Even if you spend all of that on salaries and everybody earns the same, 4k€/month for a software dev job for example seems low in central Europe. That’s not even 50k a year. Some companies offer between 60 and 80k for entry level positions. You need closer to twice that much to be remotely sustainable with 7.
You can do something similar without any addons. Firefox allows selection of multiple tabs at once out of the box, and you can have it create bookmarks for this selection. You can then have it open all bookmarks in a bookmark folder at once.