There is no modus tollens here. Your interpretation
If God loved us first, then we love Him.
is not what's written there. The verse consists of a statement/claim/fact ("We love") and an explanation ("because He loved us first").
Also, the "we love" is not about us loving God. It's about us loving each other (1 Jn 4:7; "Beloved, let us love one another, for love is from God, ..."). So your claim
But if we don't love Him (let's say were atheists), then He never previously loved us?
doesn't hold against this passage.
Of course, it is commonly said that we find God through others, and that love for God is love for one another and vice versa. But this can be interpreted as "If you love one another, you automatically love and know God" (compare 1 Jn 4:8; "Anyone who does not love does not know God, because God is love").
You can see that there is no modus tollens here in the Greek:
Ἡμεῖς ἀγαπῶμεν, ὅτι αὐτὸς πρῶτος ἠγάπησεν ἡμᾶς.
ἀγαπῶμεν love (praesens; hence a statement)