Tab Prison — Focus Tab Locker od Swalter Factories

Tab Prison locks you into the tabs you choose — and makes it physically impossible to go anywhere else.
When you need to actually get work done, open Tab Prison, select the tabs you're allowed to use, and hit Lock. From that moment, every attempt to click another tab bounces you straight back. Every attempt to open a new tab closes it instantly. The rest of your browser simply does not exist until you decide to unlock it.
On your focus tabs, a full-screen prison wall appears on any blocked tab with one-click buttons to jump back to your allowed tabs — so you never need to touch the tab bar. The extension popup becomes your control center: see your live session timer, jump between focus tabs, and end the session whenever you're ready.
Features:

Select up to 8 tabs to lock into — everything else is unreachable
New tab attempts (Ctrl+T, links opening in new tabs) are closed within milliseconds
Full-screen block wall appears instantly on any tab you're not allowed to be on
Jump buttons on the wall let you switch between focus tabs without touching the tab bar
Live session timer in the popup counts every minute of focused work
Session history tracks total focus time across all past sessions
Lifetime stats: sessions completed, tabs blocked, total focus time
One click to unlock — no passwords, no timers, no hoops

Why Firefox specifically:
Tab Prison uses Firefox's persistent background page — a real process that stays alive for your entire browser session. This is what makes the enforcement actually work. Every tab switch is caught immediately, every time, with no delays and no gaps.
Your privacy:
All data is stored locally on your device. Nothing is collected, transmitted, or shared. Tab Prison has no server, no account, and no analytics.