A Study to Assess the Efficacy and Safety of Firmonertinib Versus Placebo for Adjuvant Treatment in Participants With Stage IB - IIIB NSCLC With Uncommon Epidermal Growth Factor Receptor (EGFR) Mutations, Following Complete Surgical Resection With or Without Adjuvant Chemotherapy（FIRMOST）

Inclusion criteria

• Sign the Informed Consent Form (ICF).
• Aged ≥ 18 years old. Participants from Japan/Taiwan aged ≥ 20 years old.
• Histologically confirmed diagnosis of primary non-small cell lung cancer (NSCLC) of predominantly non-squamous histology.
• Underwent complete surgical resection of primary lung cancer and systematic lymph node dissection (R0 resection).
• 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).
• 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:

• NSCLC with EGFR Exon 19 deletion or L858R or C797S mutation.
• Incomplete resection (R1/R2) or segmentectomy or wedge resection only.
• Prior treatment with any of the following:

• Prior treatment with any antineoplastic therapy other than standard platinum-based doublet adjuvant chemotherapy.
• prior treatment with neoadjuvant therapy.
• Concurrent malignant tumors other than the primary tumor; participants with cancers that can be treated locally and cured may be eligible.
• Previous ILD (including drug-induced ILD) or active ILD/active radiation pneumonitis.