For Firefox settings: Click on the menu button in the top-right corner of the window and select "Options". In the "General" tab, scroll down to the "Tabs" section and make sure that the "When you open a link in a new tab, switch to it immediately" option is checked.

For Thunderbird settings: Click on the menu button in the top-right corner of the Thunderbird window and select "Options". In the "General" tab, scroll down to the "Links" section and make sure that the "Open new windows in a new tab instead" option is checked.

Let me know if you have any queries.

Normal Links:

• browser.link.open_newwindow
  1:current tab; 2:new window; 3:new tab;

JavaScript Links:

• browser.link.open_newwindow.restriction
  0:divert always; 1:divert never; 2:divert no features

External links:

• browser.link.open_newwindow.override.external

If this pref has the default value -1 then browser.link.open_newwindow is used. The meaning of the value of browser.link.open_newwindow.override.external is the same as of browser.link.open_newwindow

You can open the about:config page via the location/address bar. On the warning page, you can click "Accept the Risk and Continue" to open about:config.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox