。
She's made up her mind to quit her job.
Literal
She [topic-は] company [object-を] quit-let's [quotative-と] mind-decide is-doing.
~ようと決心する is a useful set pattern for committing to an action: volitional form (辞めよう = 'let me quit') + と (quotative, marking the embedded thought) + 決心する ('decide, make up one's mind'). The full chain reads as 'decide that one will V.' Add ~ている for resulting state and you get 'has decided / has made up her mind' — the decision is settled and continues to hold. 辞める is the standard verb for leaving a job, distinct from 止める ('to stop, halt') despite sharing one of its possible kanji.