I can't reproduce it on Firefox 115.20.0.

Any errors in the Console (use Inspect (Q)) ?

Test in private window; if persists, share console errors for precise fix.

Temporarily disable Firefox extensions (esp. ad/tracker blockers) and Enhanced Tracking Protection (Strict mode).

Also, Firefox strictly enforces Content Security Policy, preventing inline scripts/styles or iframes needed by these editors. This can cause delayed or failed loading if your site's CSP header lacks directives like script-src 'self' 'unsafe-inline' (use nonces/hashes for security) or child-src 'self'.

Thank you for your detailed response.

For my tests, I'm using Firefox version 148 in Windows 11. I think you could reproduce the issue with this version, as I've encountered it on several computers.

The first time I open the editing page, the editor displays normally. However, the second time I open the page, it disappears, and I have to wait several minutes, refreshing the page repeatedly, before it appears. (And so on.)

The problem occurs in private browsing mode.

The problem also occurs with "block ads" disabled.

Here are the error messages when the problem occurs. (I don't have a "cache" folder in the root directory of the site.)

Failed to load icons: default from url http://192.168.50.162/cache/1772092595/default/tinymce/icons/default/icons.min.js tinymce.min.js:10:258853
    $E http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    FB http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    FB http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
Failed to load plugin: autosave from url plugins/autosave/plugin.min.js tinymce.min.js:10:258853
    $E http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
Failed to load plugin: emoticons from url plugins/emoticons/plugin.min.js tinymce.min.js:10:258853
    $E http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
Failed to load plugin: quickbars from url plugins/quickbars/plugin.min.js tinymce.min.js:10:258853
    $E http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
Failed to load plugin: advcode from url plugins/advcode/plugin.min.js tinymce.min.js:10:258853
    $E http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
Failed to load plugin: lists from url plugins/lists/plugin.min.js tinymce.min.js:10:258853
    $E http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
Failed to load plugin: image from url plugins/image/plugin.min.js tinymce.min.js:10:258853
    $E http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
Failed to load plugin: autoresize from url plugins/autoresize/plugin.min.js tinymce.min.js:10:258853
    $E http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10
    n http://192.168.50.162/cache/1772092595/default/tinymce/tinymce.min.js:10

And I want to make it clear that the problem is not present on other browsers (Opera, Chrome, etc.). In this case, the editor opens very quickly and never presents a problem.

I don't have a "cache" folder in the root directory of the site.

The "cache" folder is a link to the "cache" folder in your data directory. Everything is OK here.

Can you repeat with CKEditor and what's in the console?

Firefox version 148 in Windows 11

Just tested it and didn't find any errors. Everything works as expected ¯\_(ツ)_/¯

The problem is more difficult to reproduce with CKEditor, but with a little persistence, I get this:

onmozfullscreenchange est obsolète. tagify.js:1:49314
onmozfullscreenerror est obsolète. tagify.js:1:49314
Le chargement du module à l’adresse « http://192.168.50.162/cache/1772186926/default/ckeditor/config/file_upload.mjs » a été bloqué en raison d’un type MIME interdit (« text/html »). 61
Échec du chargement pour le module dont la source est « http://192.168.50.162/cache/1772186926/default/ckeditor/config/file_upload.mjs ». default.mjs
Uncaught (in promise) TypeError: error loading dynamically imported module: http://192.168.50.162/cache/1772186926/default/ckeditor/config/file_upload.mjs 

CKEditor

For Apache:

Add the correct MIME type for `.mjs`:

<IfModule mod_mime.c>
    AddType application/javascript .mjs
</IfModule>

Then restart Apache (for example, `systemctl restart apache2` or `httpd`).

For nginx:

Make sure `.mjs` is mapped to JavaScript in the `http` block:

http {
    include mime.types;

    types {
        application/javascript  js mjs;
    }

    # ...
}

If you already have a `types` block with `application/javascript js;`, just add `mjs`:

types {
    application/javascript  js mjs;
}

Then reload nginx:

nginx -t
systemctl reload nginx

TinyMCE

These TinyMCE errors mean that the JavaScript files for icons and plugins are not being correctly loaded from the URLs TinyMCE expects (missing files, wrong paths, or HTML error pages instead of JS).

- Check that the files really exist

Open these URLs directly in the browser:

- http://192.168.50.162/cache/1772092595/default/tinymce/icons/default/icons.min.js
- http://192.168.50.162/cache/1772092595/default/tinymce/plugins/autosave/plugin.min.js
- http://192.168.50.162/cache/1772092595/default/tinymce/plugins/emoticons/plugin.min.js

etc

- Run 'Upgrade' via administration to clean the caches

- Clear the browser's caches

The direct links to the cache work perfectly. Otherwise, it wouldn't work with other browsers.

But I solved the problem !!!

Apparently, Firefox was experiencing an intermittent cache management issue for the editor. (in my case)

I proved it by disabling "simplecache" in the administration panel, and it worked (slowly).

So I re-enabled "simplecache" in the administration panel, and removed the "php_opcache.dll" extension from my PHP 8.4.18 server.

Now it works perfectly with Firefox !!!

Thanks, Nikolai, for your help.