Use the userchrome to true

home-manager/package-configs/firefox.nix

@@ -24,6 +24,7 @@
         "browser.uiCustomization.state" = ''{"placements":{"widget-overflow-fixed-list":[],"unified-extensions-area":["ubo-scope_raymondhill_net-browser-action","treestyletab_piro_sakura_ne_jp-browser-action","wayback_machine_mozilla_org-browser-action","plasma-browser-integration_kde_org-browser-action","kde-connect_0xc0dedbad_com-browser-action","_f209234a-76f0-4735-9920-eb62507a54cd_-browser-action","bibitnow018_aqpl_mc2_chalmers_se-browser-action","_09b14d46-21c3-4a7d-b244-e756f497935b_-browser-action","_7fc8ef53-24ec-4205-87a4-1e745953bb0d_-browser-action","_74145f27-f039-47ce-a470-a662b129930a_-browser-action","_800215d6-eff0-4a62-9268-09857c048030_-browser-action","_b3e677f4-1150-4387-8629-da738260a48e_-browser-action","_d133e097-46d9-4ecc-9903-fa6a722a6e0e_-browser-action","myallychou_gmail_com-browser-action","_b7f9d2cd-d772-4302-8c3f-eb941af36f76_-browser-action","_cca112bb-1ca6-4593-a2f1-38d808a19dda_-browser-action","gdpr_cavi_au_dk-browser-action","_54da8dcf-a015-4c80-bbb4-db7f226ac7bd_-browser-action","jid1-om7ejgwa1u8akg_jetpack-browser-action","_7c1aa46e-b74f-4325-95d2-84c07d19cdce_-browser-action","_3c078156-979c-498b-8990-85f7987dd929_-browser-action","7esoorv3_alefvanoon_anonaddy_me-browser-action","_15b1b2af-e84a-4c70-ac7c-5608b0eeed5a_-browser-action","_b9db16a4-6edc-47ec-a1f4-b86292ed211d_-browser-action","dontfuckwithpaste_raim_ist-browser-action"],"nav-bar":["back-button","forward-button","stop-reload-button","urlbar-container","customizableui-special-spring2","downloads-button","_testpilot-containers-browser-action","addon_darkreader_org-browser-action","umatrix_raymondhill_net-browser-action","ublock0_raymondhill_net-browser-action","_446900e4-71c2-419f-a6a7-df9c091e268b_-browser-action","_3579f63b-d8ee-424f-bbb6-6d0ce3285e6a_-browser-action","woop-noopscoopsnsxq_jetpack-browser-action","jid1-bofifl9vbdl2zq_jetpack-browser-action","cookieautodelete_kennydo_com-browser-action","addon_simplelogin-browser-action","jid1-aqqsmbyb0a8adg_jetpack-browser-action","unified-extensions-button","reset-pbm-toolbar-button","firefoxpwa_filips_si-browser-action","jid1-mnnxcxisbpnsxq_jetpack-browser-action"],"toolbar-menubar":["menubar-items"],"TabsToolbar":["tabbrowser-tabs","new-tab-button","alltabs-button","_c607c8df-14a7-4f28-894f-29e8722976af_-browser-action"],"PersonalToolbar":["import-button","personal-bookmarks"]},"seen":["save-to-pocket-button","developer-button","treestyletab_piro_sakura_ne_jp-browser-action","addon_darkreader_org-browser-action","_testpilot-containers-browser-action","wayback_machine_mozilla_org-browser-action","plasma-browser-integration_kde_org-browser-action","_c607c8df-14a7-4f28-894f-29e8722976af_-browser-action","kde-connect_0xc0dedbad_com-browser-action","_f209234a-76f0-4735-9920-eb62507a54cd_-browser-action","ublock0_raymondhill_net-browser-action","cookieautodelete_kennydo_com-browser-action","bibitnow018_aqpl_mc2_chalmers_se-browser-action","_09b14d46-21c3-4a7d-b244-e756f497935b_-browser-action","_7fc8ef53-24ec-4205-87a4-1e745953bb0d_-browser-action","woop-noopscoopsnsxq_jetpack-browser-action","_74145f27-f039-47ce-a470-a662b129930a_-browser-action","_800215d6-eff0-4a62-9268-09857c048030_-browser-action","_3579f63b-d8ee-424f-bbb6-6d0ce3285e6a_-browser-action","_b3e677f4-1150-4387-8629-da738260a48e_-browser-action","_d133e097-46d9-4ecc-9903-fa6a722a6e0e_-browser-action","jid1-bofifl9vbdl2zq_jetpack-browser-action","_446900e4-71c2-419f-a6a7-df9c091e268b_-browser-action","myallychou_gmail_com-browser-action","_b7f9d2cd-d772-4302-8c3f-eb941af36f76_-browser-action","umatrix_raymondhill_net-browser-action","_cca112bb-1ca6-4593-a2f1-38d808a19dda_-browser-action","gdpr_cavi_au_dk-browser-action","_54da8dcf-a015-4c80-bbb4-db7f226ac7bd_-browser-action","ubo-scope_raymondhill_net-browser-action","jid1-om7ejgwa1u8akg_jetpack-browser-action","_7c1aa46e-b74f-4325-95d2-84c07d19cdce_-browser-action","_3c078156-979c-498b-8990-85f7987dd929_-browser-action","addon_simplelogin-browser-action","7esoorv3_alefvanoon_anonaddy_me-browser-action","jid1-aqqsmbyb0a8adg_jetpack-browser-action","_15b1b2af-e84a-4c70-ac7c-5608b0eeed5a_-browser-action","firefoxpwa_filips_si-browser-action","_b9db16a4-6edc-47ec-a1f4-b86292ed211d_-browser-action","dontfuckwithpaste_raim_ist-browser-action","jid1-mnnxcxisbpnsxq_jetpack-browser-action"],"dirtyAreaCache":["nav-bar","PersonalToolbar","toolbar-menubar","TabsToolbar","unified-extensions-area","widget-overflow-fixed-list"],"currentVersion":20,"newElementCount":14}'';
         "browser.urlbar.placeholderName" = "DuckDuckGo";
         "browser.urlbar.placeholderName.private" = "DuckDuckGo";
+        "toolkit.legacyUserProfileCustomizations.stylesheets" = true;
       };
       userChrome = ''
               /*