URL Modifier by Gerbil — gerbil

URL Modifier sits between Firefox and the network. Every HTTP and HTTPS request — pages, scripts, images, API calls, WebSockets — passes through your rules before it reaches the server. Each rule matches a pattern in the URL and applies your modification text, in one of three modes:
* Before : Inserts your text immediately before the matched pattern
* Replace : Replaces the matched pattern with your text (supports $1, $2 capture groups)
* After : Appends your text immediately after the matched pattern
Rules are applied top-to-bottom in order. Patterns are JavaScript regular expressions, with automatic fallback to literal string matching if the regex is invalid.

Features
* Real-time URL rewriting — intercepts all request types: main_frame, sub_frame, script, image, xmlhttprequest, websocket, media, beacon, and more
* Three-layer interception — webRequest.onBeforeRequest + webNavigation.onBeforeNavigate + tabs.onUpdated, so even cold-start Intent URLs from external apps (e.g. Signal opening a link in Firefox) are always caught
* Case-insensitive matching — patterns match regardless of URL capitalisation
* Regex or literal — full JavaScript regex with capture group references, automatic literal fallback for invalid patterns
* Enable / disable per rule — toggle individual rules without deleting them
* Live URL tester — paste any URL and instantly see the rewritten result
* Import / Export — save and load your full rule set as a JSON file
* Toolbar icon — turns colourful when active, grey when inactive
* Persistent state — rules and active state survive browser restarts
* No data collection — all processing is local; nothing is sent anywhere