The Extended Support Release of Firefox 78 can be set not to require a valid extension signature. This obviously is risky because then Firefox will happily run extensions never reviewed/cleared/signed by Mozilla, which could be malware that steals your data. But it is an option if your extensions are critical and you accept that risk. See the last section of the following article:

Add-on signing in Firefox

There is a date-time value in the prefs.js file (editable through about:config) which determines the time when Firefox next tries to re-verify your extensions. You may need to reset that value so Firefox doesn't wait until tomorrow. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste xpi-signature-verification and pause while the list is filtered

(3) Click the trash can at the right end of the row for the app.update.lastUpdateTime.xpi-signature-verification preference to remove this preference

(3) Do a normal exit/restart of Firefox, and within about a minute Firefox should reverify your extensions

I have the regular 50 version of Firefox. This method won't work?

No, 50 was not part of an Extended Support Release. You could upgrade to the Extended Support Release of Firefox 52 if you are not ready to move up to Firefox 78.

https://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win32/uk/

If uk is the wrong locale/language code, please click the .. on that page and then click into the correct folder to find the installer.

I'm afraid to update my browser and everything else for many psychological reasons(( Maybe there is some other way or maybe the developers will take pity someday and give the opportunity it's up to us to decide at our own risk? Does the ability to launch additions manually work within a single browser session? It's not hard for me to launch them every time, as long as it works...

I'm afraid to update my browser and everything else for many psychological reasons(( Maybe there is some other way or maybe the developers will take pity someday and give the opportunity it's up to us to decide at our own risk?

I can understand being nervous since your system is older and you might have limited backups. However, old versions reached their "end of life" a long time ago, so we can't expect updates.

Does the ability to launch applications manually work within a single browser session? It's not hard for me to launch them every time, as long as it works...

Firefox has an add-on debugging feature you can use to load and run unverified extensions. However, I don't know whether Firefox 50 had that feature. It is described here: https://firefox-source-docs.mozilla.org/devtools-user/about_colon_debugging/index.html#loading-a-temporary-extension

Certificate Import Option?

Another thing you could try with Firefox 50 (and other versions of that same vintage) is to import the new certificate. The steps are in the "Manual Import Method" of my old article from the last time this happened five years ago:

https://www.jeffersonscher.com/ffu/armagadd-on_2_0.html

For Step 1, you need the newer certificate, which can be found here:

https://searchfox.org/mozilla-release/source/security/manager/ssl/addons-public-intermediate.pem

You can't save that page directly because it's HTML. Instead, you can either:

(A) Manually select, copy, paste the content from lines 1 through 41 into a plain text editor and then save as "addons-public-intermediate.pem" (or a .crt extension is fine, too).

(B) On the right side of the page, click the "R" button next to "Raw". This will load a bare certificate view that you can save normally with Ctrl+S. However, Windows likely will add a .txt extension, so when importing, you need to change the filter to "All Files" (or rename the downloaded file).

Then proceed with the other steps.

Here is a Reddit thread about a Firefox 56 user's experience with this approach: https://old.reddit.com/r/firefox/comments/1jcbdi6/omnija_file_not_working/mi47d6m/

addons-public-intermediate.pem - Should I name the file like this or can it have any name? On Reddit they write that after this their browser stopped launching.

You can name it certcert.crt if you like, the name doesn't matter as long as it matches the text of the encoded certificate.

I don't know why that user got an error about reading a configuration file. As I mentioned in that thread, the message usually refers to an Autoconfig file which most users do not have and shouldn't be affected by importing a certificate. ???

You may want to make a backup of the cert8.db or cert9.db file in your currently active profile folder. This is where the imported certificate will be stored, so you can remove the updated file and put your backup file back if something goes wrong.

To find your profile folder: Profiles - Where Firefox stores your bookmarks, passwords and other user data

I'll try this method when I'm less nervous and at my own slow pace. And the method with temporary add-ons works, at least 2 add-ons I need that don't require a reboot - Adblock Plus and visual bookmarks.

It's funny. After restarting the browser, these add-ons did not reset, but continued to work. But why?

The temporary add-ons didn't have to be reloaded through about:debugging? That's strange.

Yes. I just downloaded these 2 addons and they worked. Then I restarted the browser a couple of times, but these addons were on the list and worked.

The only other more technical option in having a much more current Firefox install is by say dual booting with a 32-bit Linux distro I say 32-bit because the chances of you having a 64-bit CPU with the ancient WinXP is slimmer and it ran slower and used more resources compared to the 32-bit WinXP.

https://www.makeuseof.com/linux-distros-with-32-bit-support/ https://www.debugpoint.com/32-bit-linux-distributions/

Slava Ukraini

The only other more technical option in having a much more current Firefox install is by say dual booting with a 32-bit Linux distro I say 32-bit because the chances of you having a 64-bit CPU with the ancient WinXP is slimmer and it ran slower and used more resources compared to the 32-bit WinXP. https://www.makeuseof.com/linux-distros-with-32-bit-support/ https://www.debugpoint.com/32-bit-linux-distributions/ Slava Ukraini

But this is really difficult for me with my current deteriorating cognitive functions and mental state...

Героям слава.

I'll try this method when I'm less nervous and at my own slow pace. And the method with temporary add-ons works, at least 2 add-ons I need that don't require a reboot - Adblock Plus and visual bookmarks.

I did everything carefully point by point. Restarted the browser and have been waiting for a long time. Nothing happens. How do I know that Firefox has definitely accepted this file with the certificate?

.....

Add-ons still not working. Need help. Didn't the browser accept this certificate?!

... Need help...

I'm sorry, that was my best guess for Firefox 50. There is no supported solution for Firefox 50 so people need to experiment as best they can.

You still have the option to install the Extended Support Release of Firefox 52 (https://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win32/uk/) and then turn off extension verification as described in the last section of this article: Add-on signing in Firefox.

But can you explain to me what is on the screenshot? Does it mean that the certificate was not accepted? And why exactly?

The screenshots look okay to me.