Troubleshoot Mode is useful when a Firefox problem can be repeated: a page draws incorrectly, an interaction fails or browsing becomes unstable. Its value comes from comparing the same task with several features temporarily disabled.

Before restarting, save work and record the failing page and action. A private test document or non-sensitive page is preferable when reproducing a problem for support. Note whether the failure occurs every time or only intermittently.

Choose the temporary test

Open Firefox’s menu, Help, then Troubleshoot Mode and accept the restart. In the next window, choose Open for the temporary comparison. Refresh Firefox is a different option that permanently resets parts of the setup; it is not required to run this test.

Repeat the original action. Mozilla identifies extensions, themes and hardware acceleration among the features disabled in this mode. If the problem disappears, that narrows the investigation, but it does not prove which individual extension or feature caused it.

Record the result before further changes

Close Firefox and start it normally to leave Troubleshoot Mode. Check whether the original behaviour returns. Record the normal-mode result and the test-mode result side by side, including any difference in the website or account used.

If the problem persists in both modes, investigate beyond the disabled items rather than repeatedly removing add-ons. If it occurs only in normal mode, change one relevant item at a time and repeat the same test. Preserve your data before making a permanent reset. An intermittent issue needs more than one successful page load before you call it resolved.

Source: Mozilla Support.