The certificate I see on https://www.pinterest.com/ was effectice as of August 13, 2015. Maybe your Firefox has an old one stored? You could check for and remove that using the Certificates Manager. Either:

"3-bar" menu button (or Tools menu) > Options > Advanced

Click the Certificates mini-tab, then the "View Certificates" button. The Certificate Manager dialog should pop up. Click the Servers tab to view the exceptions list. Please ignore the ones where the server is listed as "*" and look for any for Pinterest. If you find one, select and remove it.

If that's not it, have you tried "the usual"? When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click a blank area of the page and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"
• click the padlock or globe icon in the address bar > More Information > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page. Does that help?

The link you gave me almost 'refreshed the page' although it wouldn't let me log in. I did delete an older version from my Bookmarks, and that seemed to work. I had cleared cache prior to today, so I thought I'd save that for last, and I didn't have to bother. Thanks so much!

I tried clicking on my own boards in Pinterest and got the error again. And can't seem to shake it. I have gone through the suggestions from you above and nothing seems to be working now. Ah well, thanks for your help!

Do you know whether you have "Microsoft Family Safety" enabled on Windows 10? That may sometimes cause secure connection issues on social sites.

More info about this feature: http://windows.microsoft.com/en-us/windows-10/turn-off-microsoft-family-settings

No, that's not it. Thanks

I don't know why some pages would work in Pinterest and not others. Do you notice any difference in the website name in the address bar between the working and non-working pages?