All browsers are crashing whenever a website is opened

Part 1 of 2

The fact that Safari, Firefox, and Chrome all crash on certain websites makes it useful to determine whether the problem also occurs outside a normal macOS startup.

First, test the iMac in Safe Mode.

Since this is a 2019 Intel iMac, restart it while holding Shift until the login window appears. 

Apple explains the procedure here: Apple — Start up your Mac in safe mode

Try several websites that normally cause the crashes.

If the problem disappears in Safe Mode, that provides evidence that something loaded during a normal startup may be involved.

If the crashes continue, connect the iMac temporarily to another network, such as a phone's Personal Hotspot, and test the same websites. 

Apple also recommends checking VPNs, third-party security software, and network settings when investigating Safari problems. Apple — If Safari doesn't work as expected on Mac

Also run Apple Diagnostics by shutting down the iMac, turning it on, and immediately holding D. 

Apple Diagnostics checks internal hardware but does not diagnose macOS or other software problems. Apple — Use Apple Diagnostics to test your Mac

Finally, save a crash report from Console >> Crash Reports or ~/Library/Logs/DiagnosticReports. 

If you upload one here, we can examine it for the exception and components involved in the crash.

I would not reinstall macOS or erase the Mac until these tests have been completed.

Part 2 of 2

There are 4 Categories of Third Party Software / Services that are not needed 

➡️. Get rid of them via the Developers Instruction ⬅️

In no special order 

1 - Third Party 2 Way Firewalls 

2 -Commercial VPNs 

3 - Third Party Security Software 

4 - Third Party Disk Cleaners / Optimizer 

Since it happens in Safari, Firefox, and Chrome, I’d look beyond the individual browsers.

First make sure Sequoia and Safari are fully updated. Apple has released Sequoia updates that address crashes triggered by web content. About the security content of macOS Sequoia 15.7.5 - Apple Support

If it still happens, start that Intel iMac in Safe Mode: restart it and immediately hold Shift until the login window appears. Then try a few of the sites that consistently crash. Safe Mode is specifically useful for finding problems caused by software that loads at startup. Start up your Mac in safe mode – Apple Support

If those sites work normally in Safe Mode, I’d next look at third-party security software, web-filtering software, VPNs, ad blockers, or other background utilities installed on the Mac. That would help narrow this down quite a bit.

That’s a useful result — it points pretty strongly to something in the old user account rather than macOS itself.

I wouldn’t trash the old account yet. First make sure you have a current backup, then move personal files such as Documents, Desktop files, Downloads, etc. to the new account, using /Users/Shared if needed. I would not copy the old ~/Library folder wholesale, since that could also bring the problem with it.

Also check Photos, Mail and anything else that may contain local-only data. If iCloud is being used, sign in on the new account and make sure everything has finished syncing before removing anything.

Once the new account has everything you need and has been working normally for a while, you can remove the old account. I’d initially choose the option to preserve its home folder rather than delete it outright, just in case something was missed.

Please download and run Etrecheck.  The free version is sufficient.

Copy the report as shown in this animated screenshot

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if we can determine the cause of the problem.

Never heard this before— all web browsers acting the same way seems odd enough.

The current Sequoia is 15.7.9 —I would start there

Keep your Mac up to date - Apple Support

Keep your Mac up to date - Apple Support

if you turned on the user application Firewall— turn it OFF

To trouble shoot further you can:

—A SafeBoot https://support.apple.com/en-us/HT201262 will sort many anomalies

Does a quick disk repair before it fully boots up, and certain system caches get cleared and rebuilt, third party system modifications and system accelerations are disabled temporarily.

Login and test. Reboot as normal and test. Caches get rebuilt automatically.

This test will tell you if third party interference; most extensions etc are not loaded in safe boot mode.

*** (if you are using a wireless keyboard with rechargeable batteries— connect the charging cable, this will make the wired keyboard successful Safe Boot your Mac.)

—Test issue in another user (or guest user) account Change Users & Groups settings on Mac - Apple Support

This will tell you if it a universal issue or isolated to your user/admin account. 

unplug all non-essential peripherals when testing

Uninstall all third party apps that are Cleaners/Optimizers/VPN/Anti-Virus

all known to cause issues on the macOS

Go to Finder > Go > Go to Folder, and paste this location in there and delete the "com.apple.SpeakSelection.plist" file (don't forget the "~" character in the front):

~/Library/Preferences/com.apple.SpeakSelection.plist

That may resolve the problem of the browsers crashing when done on your original User account.