URL Modifier by 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