As Andrew says, replacing that with because leaves the meaning unchanged; but the syntax is quite different.
- The because version is classified by traditional grammar as an 'adverbial' modifying the verb got, and more contemporary syntacticians regard it as a preposition phrase modifying the clause My employees got furious.
- In this sort of context contemporary syntacticians tend to regard 'content' clauses, finite clauses optionally introduced by that, as complements of the adjective rather than as modifiers of either the verb or the clause.
The notion of complement is somewhat loose, but it will usually be applied when a particular lexical or syntactic element is 'selected' (required or permitted) by a particular term. In your example, the content clause is selected by the adjective furious, which licenses
- content clauses —
furious (that) new features were not being added
some preposition phrases, particularly those headed by over and at, and for some speakers about —
furious over our failure to add new features
furious at our failure to add new features
?furious about our failure to add new features
infinitivals having the entity modified by furious as their implied subject —
furious to find new features were not being added
But it does not license other infinitivals and preposition phrases, or gerund-participle clauses:
*furious to not add new features
*furious for us to not add new features
*furious for our failing to add new features
*furious our omitting new features