Try packaging manually

AppRun

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+#!/bin/sh -e
+
+# Firefox specific environment variables
+# https://github.com/AppImage/pkg2appimage/commit/c9b09eb557b6b0b4d914ecb82998148f38880d1d
+export SNAP_NAME="floorp"
+export MOZ_LEGACY_PROFILES=1
+
+# Find the AppDir. It is the directory that contains AppRun.
+# This assumes that this script resides inside the AppDir or a subdirectory.
+# If this script is run inside an AppImage, then the AppImage runtime likely has already set $APPDIR
+if [ -z "$APPDIR" ] ; then
+	HERE="$(dirname "$(readlink -f "${0}")")"
+	export APPDIR="$HERE"
+fi
+
+export PATH="$APPDIR/usr/bin:$PATH"
+export XDG_DATA_DIRS="$APPDIR/share/:$XDG_DATA_DIRS"
+export LD_LIBRARY_PATH="$APPDIR/usr/lib:$LD_LIBRARY_PATH"
+
+exec "$APPDIR/usr/bin/floorp"

Floorp.desktop

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+[Desktop Entry]
+Type=Application
+Name=Floorp
+Icon=floorp
+Exec=floorp %u
+Categories=GNOME;GTK;Network;WebBrowser;
+MimeType=text/html;text/xml;application/xhtml+xml;application/xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/chrome;video/webm;application/x-xpinstall;
+StartupNotify=true

package.sh

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+#!/bin/sh
+
+VERSION="11.8.1"
+LINK="https://github.com/Floorp-Projects/Floorp/releases/download/v$VERSION/floorp-$VERSION.linux-x86_64.tar.bz2"
+
+APPDIR="Floorp.Appdir"
+
+mkdir -p build
+cd build || exit
+
+echo '===# Creating AppDir #==='
+rm -rf "$APPDIR"
+mkdir -p "$APPDIR"
+
+echo '===# Donwloading Archive #==='
+wget -nc $LINK
+rm -f floorp
+tar xf floorp*.tar.bz2
+
+echo '===# Copying Files to AppDir #==='
+# AppRun
+cp ../AppRun "$APPDIR"
+chmod a+x "$APPDIR"/AppRun
+
+# Desktop Entry
+cp ../Floorp.desktop "$APPDIR"
+
+# AppImage Icons
+find floorp -name default128.png -exec cp {} "$APPDIR"/floorp.png ';'
+ln -srfv "$APPDIR"/floorp.png "$APPDIR"/.DirIcon
+
+# Other Icons
+mkdir -p "$APPDIR"usr/share/icons/hicolor/16x16/apps
+mkdir -p "$APPDIR"usr/share/icons/hicolor/32x32/apps
+mkdir -p "$APPDIR"usr/share/icons/hicolor/48x48/apps
+mkdir -p "$APPDIR"usr/share/icons/hicolor/64x64/apps
+mkdir -p "$APPDIR"usr/share/icons/hicolor/128x128/apps
+
+find floorp -name default16.png  -exec cp {} "$APPDIR"/usr/share/icons/hicolor/16x16/apps/floorp.png   ';'
+find floorp -name default32.png  -exec cp {} "$APPDIR"/usr/share/icons/hicolor/32x32/apps/floorp.png   ';'
+find floorp -name default48.png  -exec cp {} "$APPDIR"/usr/share/icons/hicolor/48x48/apps/floorp.png   ';'
+find floorp -name default64.png  -exec cp {} "$APPDIR"/usr/share/icons/hicolor/64x64/apps/floorp.png   ';'
+find floorp -name default128.png -exec cp {} "$APPDIR"/usr/share/icons/hicolor/128x128/apps/floorp.png ';'
+
+# Binaries
+mkdir -p "$APPDIR"/usr/share
+mkdir -p "$APPDIR"/usr/bin
+mkdir -p "$APPDIR"/usr/lib
+
+cp floorp/floorp           "$APPDIR"/usr/bin
+cp floorp/glxtest          "$APPDIR"/usr/bin
+cp floorp/pingsender       "$APPDIR"/usr/bin
+cp floorp/plugin-container "$APPDIR"/usr/bin
+cp floorp/updater          "$APPDIR"/usr/bin
+cp floorp/vaapitest        "$APPDIR"/usr/bin
+# Link instead of wrapper
+ln -srfv "$APPDIR"/usr/bin/floorp "$APPDIR"/usr/bin/floorp-bin
+
+# Libraries
+cp floorp/libfreeblpriv3.so    "$APPDIR"/usr/lib
+cp floorp/libipcclientcerts.so "$APPDIR"/usr/lib
+cp floorp/liblgpllibs.so       "$APPDIR"/usr/lib
+cp floorp/libmozavcodec.so     "$APPDIR"/usr/lib
+cp floorp/libmozavutil.so      "$APPDIR"/usr/lib
+cp floorp/libmozgtk.so         "$APPDIR"/usr/lib
+cp floorp/libmozsandbox.so     "$APPDIR"/usr/lib
+cp floorp/libmozsqlite3.so     "$APPDIR"/usr/lib
+cp floorp/libmozwayland.so     "$APPDIR"/usr/lib
+cp floorp/libnspr4.so          "$APPDIR"/usr/lib
+cp floorp/libnss3.so           "$APPDIR"/usr/lib
+cp floorp/libnssckbi.so        "$APPDIR"/usr/lib
+cp floorp/libnssutil3.so       "$APPDIR"/usr/lib
+cp floorp/libplc4.so           "$APPDIR"/usr/lib
+cp floorp/libplds4.so          "$APPDIR"/usr/lib
+cp floorp/libsmime3.so         "$APPDIR"/usr/lib
+cp floorp/libsoftokn3.so       "$APPDIR"/usr/lib
+cp floorp/libssl3.so           "$APPDIR"/usr/lib
+cp floorp/libxul.so            "$APPDIR"/usr/lib
+
+# Bits and Bobs
+cp floorp/omni.ja             "$APPDIR"
+cp floorp/platform.ini        "$APPDIR"
+cp floorp/update-settings.ini "$APPDIR"
+cp floorp/updater.ini         "$APPDIR"
+cp -r floorp/browser/         "$APPDIR"
+cp -r floorp/defaults/        "$APPDIR"
+cp -r floorp/fonts/           "$APPDIR"
+cp -r floorp/gmp-clearkey/    "$APPDIR"
+cp -r floorp/icons/           "$APPDIR"
+
+
+echo '===# Getting Appimagetool #==='
+wget -nc "https://github.com/AppImage/AppImageKit/releases/download/continuous/appimagetool-$(uname -m).AppImage" -O appimagetool
+chmod +x appimagetool
+
+echo '===# Generating AppImage #==='
+./appimagetool --no-appstream --comp gzip "$APPDIR" "Floorp-$VERSION-x86_64.AppImage"
+chmod +x "Floorp-$VERSION-x86_64.AppImage"