If you "unlock" saved logins by entering the Primary Password, then yes, another person sitting at your keyboard can use those logins to sign in on sites. They can't read the plain text of the passwords because they will be prompted to re-enter the Primary Password.

Here's what I suggest:

(1) To re-lock saved logins (this also prevents Sync until you enter the Primary Password again), go to the Passwords page and click the "eye" icon for any password. Then cancel/Escape the Primary Password dialog.

There really should be an easier way to do this, but this is what we have for now.

(2) Unless the person who needs to use the internet needs your bookmarks or history, create a guest profile for them to use. This also keeps their history out of your history. This has some steps...

One-Time Setup

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.

Click the "Create a New Profile" button, then click Next. Assign a name like Guests, ignore the option to relocate the profile folder, and click the Finish button.

Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.

Scroll down to Guests and click its Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account; the goal is to keep this profile clean.

Go to the Add-ons page (Ctrl+Shift+A), click Themes in the left column, and choose something like Alpenglow that looks very different from your regular theme so you can tell these windows apart.

That's pretty much it, although if you have guests often, you may want to create a new desktop shortcut for this profile. I know how on Windows, but I don't know the steps for Linux.

Accommodating Guests

Open the about:profiles page, find the Guests profile and click the Launch profile in new browser button to start them up.

What about your profile at this point? If you are not using Firefox, you could Quit in one of your profile's windows to prevent your guest from becoming confused and using your profile. Otherwise, you probably just need to tell them not to use your profile so their activity is kept separate from yours for everyone's privacy.

K, anyone who knows your primary password can access all the site passwords that you have saved in your Firefox profile.

If someone else needs to use the internet on your device, then perhaps they could use another installed browser?

It may also be possible to have separate profiles within Firefox, your normal one and a guest profile that is not connected to Sync? Then guests could use this guest profile, which would not contain your normal logins and other info, and would not ask for the primary password as it is not being Synced?

If it is possible to have two Firefox profiles, one connected to Sync the other not, then hopefully someone else can confirm this?

Thanks for your replies. Simply, I think, the best solution for me is to disconnect sync. Only use it as an update tool between my pcs and phone. I mainly now only use my Linux laptop and Android phone. I have a Windows desktop and laptop. I'm trying to migrate away from the insidious Windows world. Next is Google. But that's another story. Thanks again.