Even though this is not a Firefox issue - I found this for you :

https://asknoypi.com/switch-back-new-yahoo-mail-classic-view/

Also see these search results .......

'Hope this helps  !

Separate Security Issue, please update your Flash 29.0 r0 by removing it with this tool : If you use it Reinstall with Flash Player Version: 30.0.0.134 Current Version https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, . . . .) Note: Other software is offered in the download. <Windows Only>

Hi, if it is a matter of not being able to seethe option as it is not being shown or is invisible to you, please do this : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files , C:\Program Files(x86) & C:\ProgramData Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :

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

Reinstall with Current Release Firefox 61.0.1 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

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

I should also mention that : Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and Remove Cookies

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• https://support.mozilla.org/kb/delete-cookies-remove-info-websites-stored
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

If there is still a problem, Start Firefox in Safe Mode :

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Before using the Clean Reinstall posted 1st.