This is a Phase 3, global, double-blind, randomized, controlled multicenter clinical study to assess the efficacy and safety of adjuvant treatment with firmonertinib versus placebo in participants with Stage IB-IIIB NSCLC with uncommon EGFR mutations (exon 20 insertions, PACC and classical-like mutations) after complete surgical resection with or without adjuvant chemotherapy.

About 338 eligible participants will be enrolled and randomized in a 1:1 ratio to receive either firmonertinib 240 mg QD or placebo QD in 21-day treatment cycles. Before randomization, the participant must have undergone complete surgical resection (R0 resection), must have sufficiently recovered from surgery, and must have completed adjuvant chemotherapy (if applicable). The participants will receive firmonertinib or placebo as adjuvant treatment until unacceptable toxicity, withdrawal of informed consent, disease recurrence, initiation of new antitumor treatment, completion of treatment, or other end of treatment reason is met.

Inclusion Criteria:

1. Sign the Informed Consent Form (ICF).
2. Aged ≥ 18 years old. Participants from Japan/Taiwan aged ≥ 20 years old.
3. Histologically confirmed diagnosis of primary non-small cell lung cancer (NSCLC) of predominantly non-squamous histology.
4. Underwent complete surgical resection of primary lung cancer and systematic lymph node dissection (R0 resection).
5. Classified post-operatively as Stage IB, II, IIIA, or IIIB (T3N2M0 only) on the basis of pathologic criteria, with the disease staging following the 9th Edition TNM Staging Classification: Lung Cancer issued by Union for International Cancer Control (UICC) and American Joint Committee on Cancer (AJCC).
6. Documented results of the presence of uncommon EGFR mutations (exon 20 insertion mutations, PACC mutations, and/or classical-like mutations, either as single mutations or as co-mutations), in tumor tissue or blood via a validated NGS or validated PCR assay.

Exclusion Criteria:

A participant would be excluded from the study if he/she meets any of the following:

1. NSCLC with EGFR Exon 19 deletion or L858R or C797S mutation.

2. Incomplete resection (R1/R2) or segmentectomy or wedge resection only.

3. Prior treatment with any of the following:

   1. Prior treatment with any antineoplastic therapy other than standard platinum-based doublet adjuvant chemotherapy.
   2. prior treatment with neoadjuvant therapy.

4. Concurrent malignant tumors other than the primary tumor; participants with cancers that can be treated locally and cured may be eligible.

5. Previous ILD (including drug-induced ILD) or active ILD/active radiation pneumonitis.