Firefox: If you find the default tab grouping in Firefox to be a mess, try Tab Groups Helper. It's faster, more streamlined, and has easy keyboard shortcuts to switch between your groups.
When you have multiple tabs open and want to group them together, fire up Tab Groups Helper by right-clicking on a tab and selecting the option in the context menu. From there, you can choose to manage groups, show all groups, create a new group, or quickly start one of the groups you already have going. When you open a group, only the tabs in that group are visible, clearing up your tab bar of all other distractions.
The interface is dead simple. Each group is a column and the tabs can be dragged and dropped as you want. I like that it also has a little search function to quickly find a tab when you have too many open.
To me, the winning element was the keyboard shortcut of Alt+[number]. Your tab groups are numbered, and just as you would switch between tabs using Ctrl+[number], you can switch between tab groups with Alt+[number].
The only thing I'm missing in Tab Groups Helper is the ability to save certain groups forever, so that it acts as a session manager of sorts. But it's not a dealbreaker and the included functionality is enough to make this worth downloading.