For supporting "this" very specifically, ACLU (in the form of their 501(c)(3) foundation) is directly involved as legal counsel to Signal, so donating to EFF would be less relevant to "this" than donating to either Signal (also a non-profit) or to the ACLU Foundation. But, sure, the EFF is also a good organization.

If your concern is specifically the ACLU's disagreement with recent Supreme Courts on the intended scope of Second Amendment rights, the EFF isn't going to address that either since it's out of scope for their mission. But that's in no way relevant to what Signal had to deal with here, an area in which I think ACLU and EFF are pretty well aligned.