Firefly Aerospace Inc美股招股说明书(2026-04-21版)

11,111,116 Shares Common Stock This prospectus supplement updates and supplements the prospectus dated December19, 2025, as supplemented or amended from time to time (the“Prospectus”), which forms a part of our Registration Statement on Form S-1 (Registration No.333-291599). This prospectus supplement is being filedto update and supplement the information in the Prospectus with the information contained in our Current Report on Form 8-K, filed with the Securitiesand Exchange Commission on April21, 2026 (the “Current Report”). Accordingly, we have attached the Current Report to this prospectus supplement. The Prospectus relates to the offer and sale from time to time by the selling securityholders named in the Prospectus (the “Selling Securityholders”) ofup to 11,111,116 shares of Common Stock, par value $0.0001 per share (the “Common Stock”) of Firefly Aerospace Inc. (“Firefly Aerospace”) that werereceived by such Selling Securityholders as consideration in connection with Firefly Aerospace’s acquisition of SciTec Innovations, LLC, a Delawarelimited liability company. This prospectus supplement should be read in conjunction with the Prospectus. This prospectus supplement updates and supplements the information inthe Prospectus. If there is any inconsistency between the information in the Prospectus and this prospectus supplement, you should rely on theinformation in this prospectus supplement. Our Common Stock is listed on the Nasdaq Global Market under the symbol “FLY”. On April20, 2026, the closing price of our Common Stock was$42.79 per share. Investing in our Common Stock involves risks. See “Risk Factors” beginning on page 9 of the Prospectus, and undersimilar headings in any further amendments or supplements to the Prospectus. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities ordetermined if the Prospectus or this prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense. UNITED STATESSECURITIES AND EXCHANGE COMMISSIONWashington, D.C. 20549 FORM 8-K CURRENT REPORTPursuant to Section13 or 15(d)of the Securities Exchange Act of 1934 Date of Report (Date of earliest event reported): April15, 2026 Firefly Aerospace Inc.(Exact name of registrant as specified in its charter) 001-42789(CommissionFile Number) 2203 Scottsdale DriveLeander, Texas(Address of principal executive offices) 78641(Zip code) Registrant’s telephone number, including area code: (512) 893-5570 Not applicable(Former name or former address, if changed since last report) Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of thefollowing provisions: ☐Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)☐Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)☐Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))☐Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c)) Securities registered pursuant to Section12(b) of the Act: The Nasdaq Stock Market LLC Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of thischapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter). Emerging growth company☒ If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with anynew or revised financial accounting standards provided pursuant to Section13(a) of the Exchange Act.☐ Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; CompensatoryArrangements of Certain Officers. On April16, 2026, Firefly Aerospace Inc. (the “Company”) entered into confirmatory employment letters with each of Jason Kim, Darren Ma andRamon Sanchez (the “Employment Letter Agreements”), which supersede and replace each of the executive’s prior employment letter agreements withthe Company. Messrs. Kim’s, Ma’s and Sanchez’s Employment Letter Agreements memorialize each executive’s current annual base salary ($500,000,$420,000 and $425,000 for each of Messrs. Kim, Ma and Sanchez, respectively) and bonus opportunity with targets denominated as a percentage of basesalary (100%, 60% and 50% for each of Messrs. Kim, Ma and Sanchez, respectively), eligibility to participate in benefit plans maintained by theCompany and reaffirmation of each executive’s commitment to certain restrictive covenants set forth in the Company’s Employee ProprietaryInformation Agreement. On April15, 2026, the Board of Directors of the Company also approved certain clarifying amendments to the Firefly Aerospace Inc. ExecutiveSeverance Plan (as amended, the “Severance Plan”). In