I installed Link Password in portable Firefox 97 and I see this after installation:
Welcome to Link Password 5.0
Hi and sorry for your inconvenience with Link Password in lately.
Long story short, Firefox recently release it Firefox version 57 that totally break the functionality in Link Password and I (Link Password developer) was forced to rewrite the addon from start to working with Firefox 57.
Unfortunately some functionality that exists early has to be change in Link Password 5.0 (due to Firefox 57 changes). The Link Password is now using ext+linkpassword:// as protocol any exiting encrypted bookmark your have should be updated to ext+linkpassword:// next time Firefox restarts.
Checkout the new icon
for accessing options for encrypt and decrypt bookmarks folders.
Decrypt bookmarks from anywhere at "
Link Password - Encrypted Link or wherever you do not have access to Link Password addon.
Please leave any feedback or issue you have on
Link Password – Get this Extension for 
Firefox (en-US) and we can see what we can do.
Best Regards Link Password Developer.
It's working for me.
I click the padlock icon and enter a password to encrypt the whole bookmarks folder I want. If I export the bookmarks in JSON format the contents are encrypted. If I go back into the browser and click the padlock and enter the password the bookmarks are now unencrypted in an exported bookmarks backup in JSON format.
So my question: How are you using the extension? What are your expectations beyond what I see here that it's doing? Perhaps the extension got changed form what you were used to?
Two other questions: When did the problem start to occur and what Firefox version are you using?
You can "refresh" Firefox.
Press and hold the SHIFT key and at the same time open Firefox. Chose the option to Refresh. Settings will be reverted!
Sometimes the Prefs.js file can get messed up. This is the file that holds certain browser and add-on settings. Ever change a config/pref in about:config? This is where that setting resides. This Prefs.js file is located in your Firefox profile folder in the AppData directory in Windows. An easy way to find it is with Everything.exe by VoidTools. The Prefs.js file can be opened and edited with a text editor. I use Notepad ++. The Prefs.js file can also be backed up and then the original deleted which will be rebuilt again when you start Firefox.
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Addendum:
On the subject of real security with this add-on, I looked over the code and it uses AES 256, but with an MD5 hash. Also, the salt is derived from a simple JS math calculation derived from your machine. With encryption, true randomness is VERY important. They actually have hardware that plugs into a computer to achieve this. About the MD5 hash. This means your encrypted bookmarks can essentially be decrypted in an instant with something called rainbow lookup tables. If I was to code an add-on like this I'd have an input box for random salt generation from at least two
simple sources: A) your random mouse movements in an input box and B) your own text input. Then combine the two with other random data taken from the machine. Then add the hash of the bookmark link which should be at least Bcrypt or even better, Argon2i.
Anyway, just nerding out here. HAHAHA Pretty cool extension none the less in its concept, but its deployment is kinda a false sense of security if someone who knows what they're doing gets a hold of your encrypted bookmarks. I suppose for most people they will never be able to see what you have bookmarked. And for that I guess it serves its intend purpose.
PS:
Ah, something else I see here. Your Private Bookmarks add-on might be interfering with the Link Password add-on.
Also, I wouldn't use Adblock Plus/Adblock, and you have several types installed for certain things. You're far, far better off with the more speeder uBlock Origin. It'll accomplish what Adblock Plus does and more! Check that out, but first disable or uninstall Adblock Plus. All of them. I
think you can import an Adblock Plus list into uBlock Origin. Make sure it's the Origin version. All of the documentation is at Github and withen the add-on there are help links that'll take you to Github. If you noticed a slow browser, especially on start, it's probably Adblock doing it.
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And I can't get the Private Bookmarks add-on to work either. Looking at the add-on page the reviews from people say the same thing
here and the add-on hasn't been updated in some three years so that may be why. I'm assuming Firefox "updated" something since the last release of Private Bookmarks and now borked the add-on's functionality.
However, the Link Password add-on I linked to above will work. Just note my comments on its real security though. Like you said, you don't have high security in mind so it's a moot point I guess. Just make sure the old, and now appearing to be defunct Private Bookmarks add-on is disabled or uninstalled if you use the Link Password add-on. If you had any private bookmarks you'll need to unencrypt them first with the password before you uninstall the Private Bookmarks add-on and use Link Password add-on.
