Wordshift by BlueHarborWorks

Wordshift is a privacy-first find-and-replace extension for Firefox. Change visible text in the current tab with Quick Replace or save automatic rules that run again on future visits.

Private by design: Wordshift collects no data and sends nothing anywhere. It has no account, analytics, telemetry, advertising, cloud service, or remote server. Page text, browsing history, rules, settings, and usage stay in Firefox on your device.

Features:
- Quick changes stay active in the tab, survive reloads, work alongside automatic rules, and can be removed individually.
- Automatic rules apply on future visits and to content that loads later.
- Match anywhere in text, whole words only, or with regular expressions.
- Organize rules into groups; enable, disable, or delete rules directly from the library.
- Pause automatic rules for one site without disabling Quick Replace.
- Import and export local Wordshift JSON backups.

Wordshift landing page

Source code on GitHub