Further and furthermore are not interchangeable if you solely look at the dictionary. The definition for "furthermore" clearly concentrates on this usage, which is in contrast to the definition for "further". Miriam Webster states this definition of "further" as definition 2 and compares it with "moreover". I criticize the dictionaries' lack of examples on words such as this. For example, if you click the link for "moreover", you will shown that "besides" is a synonym. As a result, non-native English writers get in the habit of writing "Besides, ..." by itself. "Further" evolved to mean furthermore by people writing: "Further to this argument..." and now there is a situation where people think "Further, ...." and "Besides, ..." sound natural and acceptable to everyone. But just by looking at the differences of opinion here, they clearly don't. And if they did, there would be no need to have the word "Furthermore".

"Further" and "furthermore" are not interchangeable if you solely look at the dictionary. The definition for "furthermore" clearly concentrates on this usage, which is in contrast to the definition for "further". Merriam-Webster states this definition of "further" as definition 2 and compares it with "moreover". I criticize the dictionaries' lack of examples on words such as this. For example, if you click the link for "moreover", you will be shown that "besides" is a synonym. As a result, non-native English writers get in the habit of writing "Besides, ..." by itself. "Further" evolved to mean furthermore by people writing: "Further to this argument..." and now there is a situation where people think "Further, ...." and "Besides, ..." sound natural and acceptable to everyone. But just by looking at the differences of opinion here, they clearly don't. And if they did, there would be no need to have the word "furthermore".