MonoHeader by Mattias Mattsson

MonoHeader is a professional, privacy-focused HTTP header modification tool created by Monolith Consulting AB.

Create rules that modify HTTP request and response headers based on URLs, domains, request methods, resource types, and other conditions. Rules can be organized, enabled, disabled, inspected, and deployed directly from the extension.

Key features:

• Add global request headers quickly from the toolbar popup
• Modify request and response headers using Set, Append, and Remove operations
• Enable or disable individual rules without opening the full workspace
• Organize rules using profiles, groups, tags, and priorities
• Inspect conflicts and preview the effective result of matching rules
• Store sensitive header values for the current browser session only
• Import and export configurations using a versioned local format
• Review deployment history, diagnostics, warnings, and runtime status
• Configure optional per-site keep-alive behavior
• Match keep-alive sites using exact origins, subdomains, or wildcard patterns
• Exclude specific domains from broader keep-alive rules
• Use Activity pulse or an optional same-origin HTTPS request for keep-alive checks
• Test, pause, reset, and diagnose keep-alive behavior per tab

Privacy by design

MonoHeader operates locally inside Firefox. It does not send header configurations, header values, browsing activity, diagnostics, or usage analytics to Monolith Consulting AB or any third-party service.

Optional keep-alive requests are initiated only when configured by the user and are restricted to the matching website’s own HTTPS origin. All profiles, rules, presets, diagnostics, and settings remain in the browser.

MonoHeader is intended for developers, testers, administrators, security professionals, and advanced users who need transparent control over HTTP headers and session activity.