Kadenzo Character Counter for Social Media by Jan Oršula

Kadenzo Character Counter shows a live character count as you type or paste a post, and checks your text against the real limits of the major social networks at the same time.

The networks don't count the same way. X uses a weighted scheme, links always count as 23 characters, and emoji are measured by grapheme — so a post that fits on one network can run over on another. This extension does the math for all of them in a single popup, and also flags the point where your text gets folded behind a "see more" in the feed, so you know what people actually see
before they tap.

Everything runs locally in your browser. There's no sign-in, no permissions, and no data ever leaves your device — the popup only sees the text you put into it.

Full limits reference and the web version: https://kadenzo.app/tools/character-counter