When you have a bookmark widget with more than one bookmark, you can click the Open all links in tabs button in Widget Menu. Sometimes this button doesn't function properly. This is how you fix the issue.

The first time you click the Open all links in tabs button, you get a pop-up message. In it, your browser will ask you, if you trust Start.me to open pop-ups in the future. Select the option Always allow pop-ups and redirects from https://start.me.

Click Done to confirm.

Now your browser should be able to open all sites at the click of a button. It will also ask you to confirm this step, to prevent the accidental opening of many tabs at once.

If you don't see the pop-up, you can access the Pop-up settings directly.

Go there through Settings > Advanced > Privacy & Security > Site settings > Pop-ups and redirects.

Or copy and paste this address in the Omnibox: chrome://settings/content/popups.

In the Allow section, add https://start.me.

Just like in Chrome, Firefox could prevent Start.me from opening multiple tabs at once.

Go to Options > Privacy & Security. Here you'll find the section Permissions. Click to accept all pop-ups, or click the Exceptions button.

Now enter https://start.me in the Address field, and click Allow to confirm.