Can you give an example?

Does it still happen in Troubleshoot Mode?

This happens if a web page wants to open an external page in an iframe and this website prohibits this via CSP rules or via a X-FRAME-OPTIONS header in the HTTP response headers. You can right-click the frame area with the error message and see if you can use "This Frame: Show Only This Frame" or "This Frame: Open Frame in New Tab" to display this page.

thank comrades!

zeroknight, iv attached 3 pix of an example. and yes it still happens in Troubleshoot Mode. and that "firefox can't open this page" pop-up stays there and won't go away nor let me continue in gmail,

unless i reload the whole page.

cor-el, i had already come across your info while searching support for my issue,

but when i right-click the frame area with the error message i find that i cannot  use "This Frame: Show Only This Frame" 

nor "This Frame: Open Frame in New Tab" to display this page. {it remains the blank}.

also, in case this info helps:

Troubleshooting Information This page contains technical information that might be useful when you’re trying to solve a problem. If you are looking for answers to common questions about Firefox, check out our support website.

Application Basics Name Firefox Version 78.15.0esr Build ID 20210927121355 Distribution ID mozilla-mac-eol-esr1 Update Channel esr User Agent Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:78.0) Gecko/20100101 Firefox/78.0 OS Darwin 15.6.0 Multiprocess Windows 1/1 Enabled by default Remote Processes 2 Enterprise Policies Inactive Google Location Service Key Found Google Safebrowsing Key Found Mozilla Location Service Key Found Safe Mode true Crash Reports for the Last 3 Days Report ID Submitted

Firefox Features Name Version ID DoH Roll-Out 1.3.0 doh-rollout@mozilla.org Firefox Screenshots 39.0.0 screenshots@mozilla.org Form Autofill 1.0 formautofill@mozilla.org Web Compat 11.1.0 webcompat@mozilla.org WebCompat Reporter 1.3.0 webcompat-reporter@mozilla.org Remote Processes Type Count Web Content 1 / 8 Extension 1 Extensions Name Version Enabled ID Amazon.com 1.1 true amazondotcom@search.mozilla.org Bing 1.1.1 true bing@search.mozilla.org DuckDuckGo 1.0 true ddg@search.mozilla.org eBay 1.1 true ebay@search.mozilla.org Google 1.0 true google@search.mozilla.org Wikipedia (en) 1.0 true wikipedia@search.mozilla.org Bookmark Dupes 6.6 false bookmarkdupes@martin-vaeth.org Capital One Shopping: Online Coupon Tool 0.1.1137 false {aff8af88-06a9-4eee-b383-3af08c47b8c8} Ghostery – Privacy Ad Blocker 8.11.1 false firefox@ghostery.com LastPass: Free Password Manager 4.116.0.2 false support@lastpass.com Panda Safe Web 2.0.60 false {b60873b9-51aa-4566-b2fc-c16de2ec8bff} RetailMeNot Deal Finder™️ 3.0.842 false retailmenot-genie@rmn.com uBlock Origin 1.52.2 false uBlock0@raymondhill.net VirusTotal Scan 0.6.3 false {06def8b7-bcc3-4f1e-add7-7d8222ca0efc} WOT Website Security & Privacy Protection 20220920.0wot false {a0d7ccb3-214d-498b-b4aa-0e8fda9a7bf7} Graphics Features Compositing Basic Asynchronous Pan/Zoom wheel input enabled; scrollbar drag enabled; keyboard enabled; autoscroll enabled WebGL 1 Driver WSI Info - WebGL 1 Driver Renderer WebGL is currently disabled. WebGL 1 Driver Version - WebGL 1 Driver Extensions - WebGL 1 Extensions - WebGL 2 Driver WSI Info - WebGL 2 Driver Renderer WebGL is currently disabled. WebGL 2 Driver Version - WebGL 2 Driver Extensions - WebGL 2 Extensions - Uses Tiling true Uses Tiling (Content) true Target Frame Rate 60 GPU #1 Active Yes Vendor ID 0x8086 Device ID 0x0166 RAM 0 Diagnostics AzureCanvasBackend skia AzureContentBackend skia AzureFallbackCanvasBackend none CMSOutputProfile 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On what page is this happening ? Is this about a link in an email?

You can hold the Shift key when right-clicking to get the Firefox context menu.

as i had mentioned previously it happens in certain sites, such as gmail and youtube when i click on the "help" tab. please see the attachments i posted above. they used to open "help" but now only the error window. also,i dont need to hold the Shift key when right-clicking to get the Firefox context menu, just right clicking brings it up

The youtube help iframe loads for me on version 119 but fails on 78.15.0esr. Attempting to load the iframe in a new tab fails with an error in both versions and on Chromium.

Uncaught (in promise) TypeError: can't access property "parent", a is null

Installing Ignore X-Frame-Options Header allows the iframe to load on youtube.

thanks! are you sure Installing Ignore X-Frame-Options Header is safe?