Mozilla Firefox 63 is a web browser that loads websites, stores browsing data, runs web applications, and gives the user controls over tabs, cookies, permissions, and downloaded files. This archived edition belongs to a specific point in Firefox’s development rather than the current supported browser line. It is mainly useful when someone must reproduce an older site behavior, check a historical extension, or compare web-platform changes. Installing it does not make an old website safer, and using it as a daily browser exposes the machine to security defects fixed in later releases.
Tracking-cookie controls
This edition introduced a Content Blocking panel that could block third-party tracking cookies or recognized trackers. The browser checked third-party domains against a tracker list and denied matched domains access to cookies and other site storage in that context. It also restricted the Broadcast Channel mechanism that those domains could use to exchange data between browsing contexts.
The setting changes more than the advertising visible on a page. A site can place sign-in, comments, video, payment, or account widgets on another domain. If that domain appears on the tracker list, Mozilla Firefox 63 can block the stored state that the widget expects. The page may then lose a button, forget a login, or stop completing a workflow. The browser supplies a per-site exception because one global blocking choice cannot preserve every embedded service.
The Network Monitor marks tracker requests for developers who need to inspect why a resource did not behave normally. That indicator helps separate a blocked tracking request from an ordinary server failure, but it does not tell the author whether the page should depend on that third party.
Closing and restoring
Mozilla Firefox 63 no longer displays the familiar Save and Quit choice in this release. Mozilla Firefox 63 can still restore the previous session, but the user must enable the restore setting or invoke the restore command after reopening. A warning can appear when a quit action would close several tabs or windows.
This change matters on machines used for long research sessions. Closing the browser and assuming that every tab will return can lose the working set if session restoration is off. Private windows also do not become part of a normal restored session. Bookmarks remain the safer record for pages that must survive a crash, profile reset, or cleanup of browsing data.
Developer inspection
Mozilla Firefox 63 developer tools include an Accessibility Inspector by default. It exposes the accessibility tree that assistive technology receives, which may differ from the visible document structure. The Fonts panel can edit font properties during inspection, and the Network Monitor can identify tracker requests alongside ordinary resources.
Web authors also gained enabled implementations of Shadow DOM and Custom Elements. Those technologies let a site define component boundaries and custom HTML elements. A page built around later behavior may still fail in this archived engine, however, because later standards work, certificate changes, and JavaScript capabilities are absent. A test against this edition should therefore answer a historical compatibility question, not replace testing in a supported browser.
Isolated historical use
Mozilla Firefox keeps history, cookies, saved form data, extensions, and preferences inside a profile. Using a separate profile for historical testing prevents old extensions or downgraded site data from mixing with a current profile. It also makes removal easier after the test.
The browser should stay away from primary email, financial accounts, administrative panels, and routine downloads. An archived installer preserves old behavior, but its age also preserves known browser vulnerabilities. Running it in an isolated virtual machine with disposable data limits the consequences when an old page, extension, or download behaves unexpectedly.
