Psalm 78/Notes/Grammar.v. 51.975339
For the emendation אוֹנָם in place of the MT's plural א֝וֹנִ֗ים, see the "their labors/pains" in the LXX (τῶν πόνων αὐτῶν); TgPs (צעריהון); "their strenth" in Symmachus (δυνάμεως αὐτῶν); and "their children" in the Syr. (ܝܠܕܗܘܢ). A number of Hebrew manuscripts may also read אונם (see de-Rossi, Variae Lectiones, vol. 4, 54). The preferred reading (as the MT), however, is found in Hebr. (albeit singular) principium partus "first born"; and "of pains" in Aquila's λυπῶν and Quinta/Sexta's ὀδύνων (<a rel="nofollow" class="external free" href="https://septuaginta.uni-goettingen.de/catalogue/Ra_1173/">https://septuaginta.uni-goettingen.de/catalogue/Ra_1173/</a>).
For the suffixed-form, cf. the NJPS "the firstfruits of their vigor" (see also the ESV; REB).
The plural אוֹנִים is unambiguously attested in Isa 40:26, 29 and Prov 11:7, so does not pose a grammatical problem for the MT here, of which the Hebrew manuscript evidence is otherwise very strong across both the larger Tiberian Codices and the Babylonian manuscripts. The versions may also have been influenced by וַיַּ֣ךְ כָּל־בְּכ֣וֹר בְּאַרְצָ֑ם רֵ֝אשִׁ֗ית לְכָל־אוֹנָֽם׃ in Ps 105:36.