This study is a single-center, single-arm, open-label, exploratory clinical trial. A total of 30 patients with diffuse cutaneous systemic sclerosis (dcSSc) will be enrolled. A historical control cohort will be established to evaluate the efficacy and safety of Firsekibart by comparing with historical data.

Inclusion Criteria:

1. Age 18-70 years (inclusive), male or female.
2. Diagnosis of systemic sclerosis (SSc) according to the 2013 ACR/EULAR diagnostic criteria.
3. Disease duration of diffuse cutaneous systemic sclerosis (dcSSc), as defined by LeRoy & Medsger (2001), of ≤ 5 years (from the time of first onset of non-Raynaud's phenomenon).
4. Modified Rodnan skin score (mRSS) ≥10;
5. Voluntarily signed informed consent form and ability to comply with the requirements of the study protocol.

Exclusion Criteria:

1. Allergy to the active ingredient of Firsekibart or any of its excipients, or a history of allergy to monoclonal antibodies.
2. Presence of any rheumatic disease other than SSc.
3. Moderate to severe lung disease with FVC < 60% or DLCO < 50% of predicted value.
4. Use of medications that may interfere with the evaluation of the efficacy and safety of Firsekibart, except for stable use of permitted concomitant therapies that have been maintained for at least 4 weeks prior to screening and are kept at a stable dose throughout the study period.
5. Use of biological agents or stem cell therapy within 3 months prior to screening or within 5 half-lives of the known drug.
6. Receipt of live or attenuated vaccines within two months prior to screening.
7. Severe hepatic impairment, renal impairment, or hematologic abnormalities at screening.
8. Acute or chronic infection (excluding infection complicated by finger ulceration), active infection, history of malignant tumor, or immunodeficiency disorder.
9. Women who are pregnant or breastfeeding, or subjects planning to become pregnant during the study period.
10. Any other conditions that, in the investigator's judgment, render the subject ineligible for this trial.