What is the exact error message?

This Connection is Untrusted

You have asked Firefox to connect securely to www.website.tld, but we can't confirm that your connection is secure. What Should I Do?

If you usually connect to this site without problems, this error could mean that someone is trying to impersonate the site, and you shouldn't continue."

This is followed by a "get me out of here" button and a "technical details" link. Very occasionally there's also a "I understand the risks" link which allows me to continue, but this is rare.

If I disconnect from the Adguard VPN then everything works normally.

Adguard are boasting on their website that 'you will be safe from online trackers and analytics systems'. In order to do this they'll have to able to see all traffic from your phone or browser. As many websites uses TLS encryption now, that encrypted traffic is invisible to Adguard.

My best guess what they are doing to circumvent the end-to-end encryption of your browser is to sneak in their own certificate, and establish themselves as a 'man in the middle'. This is similar to what anti-virus software does on desktop computers to 'protect' them. Since Firefox doesn't know about the Adguard cert, you get the error message.

You can try this: In the Adguard app, go to Settings --> HTTPS Filtering and turn it off.

If that doesn't work, check with Adguard on how to install their root cert into Firefox. I'm not sure that this is possible for Firefox for Android though.

IMV their approach is very questionable, as you'd need to have a lot of faith and trust in Adguard. After all they can see all your traffic in the clear, including any passwords you send when logging in to a secure website.

You may look for another VPN instead, and install an adblocker for Firefox with corresponding filter lists. https://addons.mozilla.org/android/addon/adblock-plus/ https://adblockplus.org/en/subscriptions

Many thanks! Turning off https filtering has done the trick for the few sites I've checked. Having paid more attention this time to what the settings say (I'm just assessing the free version of Adguard atm), I don't think it's 'sneaking' any certificates through - it's quite up-front about it!

My last reply somehow got truncated. It also said:

"There is an option to add the certificate to Firefox, but only in the premium version.

I've been using Adblock Plus for some years but was attracted by Adguard because it blocks third party ads in apps too. For the short time I was allowed to test the premium version this seemed to work well, but I shall take account of your comments about privacy issues when I finally decide which wary to go.

Thanks again for taking the time to answer my question - much appreciated!"