Please Refresh but do this 1st:

• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer
• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

then

• https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

TEST....... If no issues then Extensions which need to be added back in 1 at a time and tested ..... Or it is your Profile : Make a new one and test ...:

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

If is your Profile :

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Note: Any customization will revert back to default, you will also need to reinstall Extensions.

Please let us know if this solved your issue or if need further assistance.

Try to ask advice at the Stack Overflow forum site.

• http://stackoverflow.com/questions/
• https://support.mozilla.org/en-US/kb/where-go-developer-support

See also;

• https://hacks.mozilla.org/2014/02/live-editing-sass-and-less-in-the-firefox-developer-tools/

Is the problem with Firefox's Developer Tools not working according to the documentation?

https://developer.mozilla.org/docs/Tools/Debugger/How_to/Use_a_source_map

If so, maybe try this forum: https://discourse.mozilla.org/c/devtools

I performed the steps in the first reply. It did not help. Earlier I reverted to Firefox 52.x and verified that source map lookups were working in that version.

This is clearly a bug. The code that generates the URL to load the source file is creating a bad URL. It's putting the querystring in twice, and one of those places isn't even at the end of the URL. This bug makes the entire rest of the URL look like it's all part of one big querystring, which includes the filename of the source file. It gets swallowed by the buggy premature querystring insertion.

Did you go to Stackoverflow

If you have a bug, file a bug report. https://bugzilla.mozilla.org/ Bug Writing Guidelines : https://developer.mozilla.org/en-US/docs/Mozilla/QA/Bug_writing_guidelines

Please let us know if this solved your issue or if need further assistance.