If you want to search a website for text you see in a message in Thunderbird, you can instantly run the search from Thunderbird.
Let’s say you see the phrase "I like Minotaur" and want to search the web for Minotaur. Just highlight the word "Minotaur", then right-click and select Search Bing for "Minotaur"
Thunderbird will show the search results in your default browser.
By default, Bing is set as the search engine. To change it, do the following:
Google is not included in the search engine list, but you can add it by installing the Google Search for Thunderbird add-on.
After you install the add-on, Google will be available in the list of search engines.