Which form should I use?

If the time of the fireman's decision is situated some time in the past (relatively long time in the past) then you should use "had" because the past perfective is used for times (had accepted) before some time in the past (decided).

• The fireman decided to return to work tomorrow because the union had accepted his offer.

• The fireman decided to return to work tomorrow because the union accepted his offer.

From CGEL § 4.24, p. 195 : The past perfective

The past perfective has usually the meaning of "past-in-the-past", and can be regarded as an anterior version of either of the present perfective or of the simple past.
[…]
But of course, the past perfective does not have to refer to a more remote time than that referred to by the simple past. In some case, particularly in a clause introduced by after, the two constructions can be more or less interchangeable.

• I ate my lunch after Sandra had come back from her shopping.
• I ate my lunch after Sandra came back from her shopping.

However, if the time of the acceptation is relatively recent you should use has or as well the past simple.

• The fireman decided to return to work tomorrow because the union has accepted his offer.

This possibility of using the present perfective stems from the characteristic of this tense to connote states leading up to the present.

Lots of useful reforms but no revolution - The AustralianFeb 5, 2019 — ... mortgage broker sector was rejected because the government has accepted industry arguments that brokers help the smaller banks compete ...

2005 Federal Equal Opportunity Reporter - Page 906 In addition , the district court concluded that it did not have jurisdiction because the plaintiff " has accepted the remedies afforded him in the administrative process " ( emphasis added )

Aug 7, 2013 Is an Affair Ever 'Acceptable'? | Next Avenue — Yet there was little fallout for Klum (and even Sanford), apparently because the public has accepted that these were genuine love-based ..

COMMONWEALTH OF KENTUCKY BEFORE THE PUBLIC ... but did not propose exclusion of these items because the Commission has accepted this.

Sep 11, 2010 - NFL: PA is asking its members to consider decertification ... — The person spoke on the condition of anonymity because the union has not announced its plans.

Feb 19, 2016 - United Public Workers' lawsuit to stop Maui hospital ... — U.S. District Judge Helen Gillmor dismissed the case because the union "has failed to state a claim based on a violation of the contract clause ..

Jan 15, 2019 - NLRB general counsel issues new advice memoranda ... — The NLRB decided there was no duty to provide the information because "the Union has failed to identify any provision in the TCJA obligating ...

In your sentence, is found the adverb "tomorrow", which tends to show the context to be that of a state leading to the present. However, imagining a variant in which the fireman will not finally go back to work because subsequent to the acceptation a counterstatement was issue by the union.

• The fireman decided to return to work tomorrow because the union had accepted his offer; unfortunately, they cancelled their agreement shortly after the fireman's decision. (There is no state leading to the present.)