Were communists opposed to Jews in France?

Communism in general is atheist. Marx famously opined that religion is the "opium of the people." Note that different countries had different degrees of tolerance. Some have freedom of worship but don't allow organized religion (e.g. Vietnam).

Another thing to note - it can be used as a tool to manipulate people, which is what Stalin did with Eastern Orthodoxy.

That being said, it may not directly answer your question, but one can deduct that Communists were not all accepting of zealots, but tolerated religion in general as it could be politically useful.

No, at least not during World War II.

Yes, the Communist resistance was fairly late in starting (1941), but when it did, its leader, Colonel Gilles (aka Joseph Epstein), was Jewish.

Also, Leon Blum, the pre-war French premier, was both Jewish and socialist. He would not have gotten in without Communist support.

I know that the Dreyfus affair was an important changepoint for the communists and the Left for them to stand with the Jews to the extent that it was about defending human rights.

The question is partially answered by addressing chronological and terminological ambiguities. The Dreyfus affair developed from 1894 (conviction of Dreyfus) to 1906 (final acquittal.) The French Communist Party was founded in 1920 by the Comintern. Not only hadn't been there an organized Communist movement in France during the affair and the associated rise of the antisemitism ^1, but most of those identifying themselves as Communists were not these in the usual Marxist sense, but rather reflecting the pre-Marxist roots of Marxism or as a reference to the Paris Commune^2.

While in most countries Communist movements were created by (or with the aid of) Comintern and became associated either with the Moscow orthodoxy or at least with Marxism, there are some notable exceptions: France, Britain, and Germany. France and Britain had their own Socialist movements well before Marxism, and it was Marx' stay in France that inspired his ideology rather than the other way round.^3 In this sense the French left wing was very heterogeneous, and this includes its attitudes towards Dreyfus.

Further, it is necessary to point out that there were three major forces in the French political spectrum at the time of Dreyfus: the conservatives (aristocracy, monarchists, Catholics, etc.), the republicans (i.e., the liberals), and the socialists. The republican current was the ruling one, while the socialists of all kinds were in opposition to both the conservatives and the republicans. Dreyfus was associated with the Republicanism, and therefore not considered as a part of the Socialist cause. Furthermore, as Dreyfus came from a wealthy family, there was no much sympathy for him among the Socialists.

There were some notable exceptions to this, most of all:

• Léon Blum, who was himself Jewish and an active dreyfusard. He was however too young to exert any substantial influence on the affair.
• Jean Jaures, who had a change of mind from initially approving condemnation of Dreyfus to eventually supporting his acquittal and championing this cause in press and as the leader of the French Socialist Party.

(The politics at the time of the Dreyfus Affair is described in detail in Jean-Denis Bredin, L'Affaire (in French).)

Remarks
^1 It is more appropriate here to talk about the emergence of antisemitism, as an opposition to the race/people. This is distinct from anti-Judaism which was discrimination of Jews on the basis of their religious beliefs - a phenomenon that was supposed to had disappeared after the Church was separated from the state, the Jews were given equal rights, and religious beliefs became a private matter.

^2 Paris Commune was an ostensibly non-Marxist affair, following mostly the ideas of Proudhon and Blanqui. It was made into a cause célèbre of Marxism as an example of spontaneous proletarian uprising and the revolutionary terror (aka Dictatorship of the proletariat.)

^3 In Germany the Socialist movement developed alongside Marxism, mostly inspired by Ferdinand Lassalle and his followers. While the two movements co-existed and even for some time within the same Social Democratic Party of Germany, it was eventually the Lassalean movement that became entrenched - believing in peaceful transformation to Socialism, rather than a violent overthrow of the existing order (see Revisionism). The party eventually adopted Liberal views and abandoned the goal of replacing Capitalism with Socialism in its Godesberg program in 1959.