Tabstinence by Gautam Panakkal

Control how many tabs you can have open globally and/or per window, and how many windows you have open. Also, set tab limits per-website and choose websites to exclude from limits.

More details:
- Existing tabs and windows are never closed. Limits apply as tabs and windows open, so setting a limit below what you already have leaves every window and tab exactly as it is; the limit takes effect the next time you open one.
- Browser internal pages are exempt.about:addons, about:config, extension pages, and the like are never counted and never closed. about:blank, about:newtab, and about:home are treated as ordinary tabs, so plain new tabs can still be blocked.
- Tabs and windows you left open all come back at browser startup. Lazily restored tabs are preserved as well.
- Unloaded tabs are safe to open. A tab Firefox is holding without a page behind it — restored from your last session, or dropped while idle to save memory — loads the URL it already had when you switch to it. That load is not treated as a new tab, however long the tab sat unopened, so limits only apply again once the page is up and you navigate somewhere else.
- A window's last tab is never closed by a navigation, so a limit cannot make a window disappear while you are using it.